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.
Not a big deal, but it seems like it might be a little nicer to just remove the ExtensibleGlobals attributes from the config in the outer `LoadExtensibleSection` function, rather than creating another ExtensibleGlobals attribute in here just so we can figure out which fields to ignore.