Nameko-sentry

* Switch to semantic versioning
* Hardcodes Eventlet transport rather than using deprecated DSN prefix. Closes 14

* Explicitly clears Raven context on worker teardown, in case there are any references to threads in the breadcrumbs (which will cause the thread-locals to remain in memory). Fixes 12

- Makes use of Raven's contexts when reporting to sentry. Now populates:
- HTTP context with the request details, if using the http entrypoint
- User context if any keys in nameko's `context_data` seem to identify a user
- Tags context for call IDs, service and method names
- Extra context for everything else in nameko's `context_data`
- Degrades nicely when the config file doesn't contain a `SENTRY` section at all

- Reverts to using raven's own greenthread if an eventlet transport is specified in the DSN
- Separates DependencyProvider logic into more-easily overridable methods (e.g. `build_tags`, `format_message`)
- Adds a configuration key to disallow reporting of "expected" entrypoint exceptions to sentry

- Adds compatibility for Python 3
- Explicitly manages the background thread with nameko, rather than using raven's implemention