Call extension method as an extension (closes #27)

I named the analyser and the fix provider and the test with suffix CallExtensionMethodAsExtension.
 
In the tests I've tried to cover the following scenarios:
* An method extension. (No warn case)
* An static call, in order to trigger a fix  
* A static call with no parameter
* A static call with a parameter
* A static call without parameter and a full namespace declared.
 
Because this is my first contribution in an open source project, please, let me know what I need to do if any improvement on that code is necessary

Thanks