Configurable Logger

Allows Jekyll's logger to be set to any [Logger](http://www.ruby-doc.org/stdlib-2.1.2/libdoc/logger/rdoc/Logger.html) compatible instance. 

* `Stevenson` now inherits from `Logger` and extends some methods to restore passing tests.
* New `LogWriter` class provides an interface between Jekyll and Logger to maintain Jekyll's logging API.

When using Jekyll as a library this allows you to change the logger, for example to output to a file or whatever you want to do so long as it's compatible with Loggers API.