Initial logging proposal.

C.f. #112

Add logging to the Mustache Engine.

 * Add a Logger interface.
 * If a Logger instance is passed to the Engine constructor
   (or added later via setLogger) template compiling,
   caching, errors and missing partials will be logged.
 * Add a Stream Logger and Monolog Logger implementation. You
   should use the Monolog Logger.
Add get/setLevel to AbstractLogger.
MonologLogger should not extend AbstractLogger.
Handle prematurely closed streams in StreamLogger.
Update loging to match (proposed) logging PSR.
Update logger to implement accepted PSR log spec

Add tests!
Add plug for PSR-3
Merge pull request #116 from bobthecow/feature/logging

Add logging