Add circular buffer support to the Lua sandbox
Allow inject_message() to provide some metadata about the payload

- payload_type, defaults to "txt"
- payload_name, defaults to ""
Add dashboard output for better visibility into Heka

- The dashboard outputter collects data for the dashboard, generates static files,
and provides an HTTP file server to access them remotely.
    - Reports on all running Heka plugins
	- Reports on all terminated Heka sandbox plugins
	- Provides a graphical display for any Sandbox plugin that outputs a circular buffer

- flatten the sandbox filter and manager configuration (remove settings)
Merge branch 'dev' into features/circular_buffer
Correct the configuration examples in the docs

-  Remove the sandbox settings section
Remove sandbox settings configuration example (last one)
Add defaults to the dashboard configuration
Merge pull request #121 from mozilla-services/features/circular_buffer

Features/circular buffer