Bunyan

- [issue 401] Improved performance when using disabled log levels.

- [issue 454] Fix `src` usage with node v7.

- [issue 450] Fix `log.info(null)` crash that resulted from 426 in v1.8.2.

- [issue 449] Bump dtrace-provider dep to 0.7.0 to help avoid deprecation
warnings with node v6 in some cases.
- [issue 426] Ensure `log.info({err: err})` results in a "msg" value, just
like `log.info(err)`.

- [pull 386] Fix bad bug in rotation that could cause a crash with
error message "cannot start a rotation when already rotating"
(by Frankie O'Rourke). The bug was introduced in 1.8.0.

Note: *Bad release.* An addition in this release broke 'rotating-file' usage.
Use 1.8.1 or later.

- [issue 370] Fix `bunyan -p ...` (i.e. DTrace integration) on node
4.x and 5.x.
- [issue 329, pull 330] Update the 'rotating-file' stream to do a file
rotation on initialization if the mtime on the file path indicates the
last rotation time was missed -- i.e. if the app wasn't running at the
time. (by Paul Milham.)