Add Rollbar handler.
Add composer suggest for Rollbar package.

Update documentation.
Inject rollbar handler.

Typecheck exceptions.
Coding standard fix.
Flush roll bar notifier on close rather than destruct.

Inherit DocBlocks.
Include context data as extra data sent to Rollbar.
Include level, channel, and datetime in extra data sent to Rollbar.
Send datetime as a string so it can be converted to JSON.
Merge pull request #332 from PaulStatezny/rollbar-handler

Add Rollbar handler