feat(logging): use mozlog with heka format

This uses mozlog to make sure all our logs match the Heka JSON Schema.

Closes #156
BREAKING CHANGE: Both the config and the output for logging has changed.
    Config can be removed, as the defaults are what should be used in
    production.
Merge pull request #74 from mozilla/mozlog

feat(logging): use mozlog with heka format
r=rfk