Made LibLog PCL compatible. Related to issue #9.

Thanks for the great work on this project! I've found it really useful.

I needed PCL compatibility for a project at work. I've made a few changes for this.

* `#define PORTABLE` omits the `GetCurrentClassLogger()` method from the PCL version because this uses `StackFrame`, which is not available. This is commented out by default.
* No longer references `System.Diagnostics.TraceEventType` directly. Uses reflection instead.
* No longer references `System.Console` directly. Uses reflection instead.
* No longer references `System.ConsoleColor` directly. Uses reflection instead.

Please let me know if you think this is a sensible method of achieving PCL support.

It could make it smoother for nuget consumers if there were two versions of the file in the package. 

* One with `#define PORTABLE` **included** for portable targets
* One with `#define PORTABLE` **omitted** for .NET targets

This is beyond what I needed though!