Convert config loading to TOML, refactor appropriately

This PR refactors SimplePush to use TOML config loading, changes the existing portions to a new Application based init system, and config loading for each section similar to heka's.

A variety of Go idiom portions were also cleaned up, the logger was factored into an extensible logger with a uniform interface.

Fixes Issue #35.

Awaiting r+ from @jrconlin, @rafrombrc.
I'm not sure which 'global' you're referring to here. As ClientCount() needed to lock the map just to get the count though, it'd be more efficient to use an int32 with atomic operations, so I replaced the implementation of ClientCount to do that.