ILogger interface added

* ILogger interface extends ILoggerBase and ISuppress. At the moment, it
is not CLS compliant because of the version 1 (V1) interface methods.
* ILoggerBase exposes the basic Log() methods.
* ISuppress interface added. It contains the Swallow() and
SwallowAsync() methods definitions. Provides an interface to execute
System.Actions without surfacing to the calling method any exceptions
raised for that action.
Merge branch 'ILogger' of github.com:ie-zero/NLog into ie-zero-ILogger
Merge branch 'ie-zero-ILogger' into v4.0