Add enabled attribute to logger configuration

closes #13
Support optional parameters in [ConditionMethod] methods. Optional parameters can be omitted from the end of the parameter list (only).
Added optional ignoreCase argument to string-related ConditionMethods, retaining original behaviour as the default. Added new strequals function for string comparison with optional ignoreCase argument.
Fix compilation for silverlight
Implement optional parameters that work for .NET 2.0 and above, and Silverlight.
Removed contributor copyright line.
Avoid Win32-specific file functions in Mono where parts are not implemented.

Add && !MONO compiler switch around Win32-specific file functions in BaseFileAppender.cs
Specifically, WindowsCreateFile() calls Marshal.GetHRForLastWin32Error() which is unimplemented
(in Mono 3). Without this fix, many log messages are not emitted. The try/catch block added to
TryCreateFileStream() does NOT fix this.
Merge pull request #219 from KeithLRobertson/master

Avoid Win32-specific file functions in Mono where parts not implemented.
Merge pull request #216 from zanyants/master

Optional ConditionMethod arguments, ignoreCase argument for standard condition methods, EventLogTarget enhancements