Rewrite extension architecture.

- Add new header to drone request to simplify differentiation.
Rewrite readme.

- Add Name constant to drone.
Fix horrible documentation.
Fix example typo in documentation.
Re-add coveralls support.
Fix bug where handler was called multiple times.
Fix a bug where only one extension was loaded.
Update docs. Run goimports.
Add knivey's github plugin, even if it's named badly.