Added a README with basic info about installing and usage
Made isEnabled a PartialFunction

This allows specifying only the levels to enable, when configuring
a Logger.
If a level is not specified is disabled by default, and this is
determined by the isEnabledDefault value in LoggerMacro.
Added test suite

The current test suite tests whether the write method of the underlying
implementation is called correctly depending on whether each level is
enabled.