Replace strict Monolog dependency with PSR\Log Interface

While Monolog is still being used internally, the dependency has
been switched to PsrLoggerInterface.  This allows any PSR\Log
compliant logger to be used without any code modification

Closes #22
Add EmptyLogger

A logger that doesn't log.  Basically the same as Monolog's
NullHandler, but without the overhead of loading all of Monolog's
classes just to eat the response.
Remove unused imports
Fix legacy logging method call
Add easy method to enable/disable logging
vfsStream dep should not be pointed at dev master
Disable logging by default
Update documentation about changes to Logging + PSR\Log
Update yaml tests