Allow Jekyll's logger to be set to any Logger compatible instance

* Stevenson now inherits from Logger and extends some methods to use $stderr
for log messages greater than info level.
* LogWriter provides an interface between Jekyll and Logger to maintain API.
Renaming LogWriter to LogAdapter and adding tests for class

String coloring moved to Stevenson as not responsibility of LogAdapter
Rocket style hash and switching integers for Logger severity constants
Merge pull request #2444 from createdbypete/configurable-logger