Bunyan

- [issue 3] More type checking of `new Logger(...)` and `log.child(...)`
options.
- Start a test suite.

- [issue 2] Add guard on `JSON.stringify`ing of log records before emission.
This will prevent `log.info` et al throwing on record fields that cannot be
represented as JSON. An error will be printed on stderr and a clipped log
record emitted with a 'bunyanMsg' key including error details. E.g.:

bunyan: ERROR: could not stringify log record from /Users/trentm/tm/node-bunyan/examples/unstringifyable.js:12: TypeError: Converting circular structure to JSON
{
"name": "foo",
"hostname": "banana.local",
"bunyanMsg": "bunyan: ERROR: could not stringify log record from /Users/trentm/tm/node-bunyan/examples/unstringifyable.js:12: TypeError: Converting circular structure to JSON",
...

Some timing shows this does effect log speed:

$ node tools/timeguard.js before
Time try/catch-guard on JSON.stringify:
- log.info: 0.07365ms per iteration
$ node tools/timeguard.js after
Time try/catch-guard on JSON.stringify:
- log.info: 0.07368ms per iteration

- Use 10/20/... instead of 1/2/... for level constant values. Ostensibly this
allows for intermediary levels from the defined "trace/debug/..." set.
However, that is discouraged. I'd need a strong user argument to add
support for easily using alternative levels. Consider using a separate
JSON field instead.
- s/service/name/ for Logger name field. "service" is unnecessarily tied
to usage for a service. No need to differ from log4j Logger "name".
- Add `log.level(...)` and `log.levels(...)` API for changing logger stream
levels.
- Add `TRACE|DEBUG|INFO|WARN|ERROR|FATAL` level constants to exports.
- Add `log.info(err)` special case for logging an `Error` instance. For
example `log.info(new TypeError("boom")` will produce:

...
"err": {
"message": "boom",
"name": "TypeError",
"stack": "TypeError: boom\n at Object.<anonymous> ..."
},
"msg": "boom",
...

- Add `new Logger({src: true})` config option to have a 'src' attribute be
automatically added to log records with the log call source info. Example:

"src": {
"file": "/Users/trentm/tm/node-bunyan/examples/src.js",
"line": 20,
"func": "Wuzzle.woos"
},

- `log.child(options[, simple])` Added `simple` boolean arg. Set `true` to
assert that options only add fields (no config changes). Results in a 10x
speed increase in child creation. See "tools/timechild.js". On my Mac,
"fast child" creation takes about 0.001ms. IOW, if your app is dishing
10,000 req/s, then creating a log child for each request will take
about 1% of the request time.
- `log.clone` -> `log.child` to better reflect the relationship: streams and
serializers are inherited. Streams can't be removed as part of the child
creation. The child doesn't own the parent's streams (so can't close them).
- Clean up Logger creation. The goal here was to ensure `log.child` usage
is fast. TODO: measure that.
- Add `Logger.stdSerializers.err` serializer which is necessary to get good
Error object logging with node 0.6 (where core Error object properties
are non-enumerable).

- Spec'ing core/recommended log record fields.
- Add `LOG_VERSION` to exports.
- Improvements to request/response serializations.