Baseplate

* Fix string coercion for binary annotations in tracing
* Add Percent helper in config module
* Add local span support for tracing

- Handle cassandra statements when setting statement span tag
- Add `string_types` to baseplate._compat

- fix binary annotation passing in tracing rpc

- add weighted lottery implementation
- add service discovery helpers
- add filename -> file config parser
- parameterize ThriftPool's transport factory
- add more configurables to Baseplate.configure_tracing
- fix client annotations in tracing
- add instrumentation for pymemcached

- add zipkin-compatible request tracing to spans

- fix issue where we were always setting 'trace' to True in the
CassandraSessionAdapter