Helps with pattern matching.

I didn't want the lager_log_message record being used across application
boundaries, this will insulate other applications from any changes to
the message type's internal structure.

Had to add a pseudo-ternary operator to the formatter to support this.
Also allowed lager:log to log metadata.

Changed the messages sent to the backends to include metadata and separated formatting from the backend.  Added documentation, fixed tests, and removed some unused code.

Additional changes and formatting by Andrew Thompson

Use lager_trunc_io:format() for all messages submitted via
lager:log().  Also, refactor list flattening and final newline
stripping into shared func that uses re:replace() to do both.

IBM punchcards rejoice! Basho supports your right to continue to affect
how we format source code :)

az558