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.
Update README.rdoc