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.
NIT: There are a lot of instances where the error simply propagates up. Go errors usually don't have a lot of good context. It might be useful to add logging at the error sources so that we have a clearer idea of what's actually causing the problem.