Service Refactor

Resolves #38, #51, #96, #139, #142, #184 

* split `tron.service` to `tron.core.service` and `tron.core.serviceinstance`
* improved unit test coverage of the service monitoring code
* refactor of the `tron.event` module
* split `tron.www.api` into `tron.www.controllers`
* lots of general cleanup
* introduce `ConfigContext` to cleanup config parsing interface
* remove support for action `requires` as a string, which has been deprecated since 0.3.3
* remove `zap` service command, which was a hack for broken code