register CodeGenerationService in OmniSharpRefactoringContext

fixes ##114

See here:
![screenshot 2014-11-29 16 19 05](https://cloud.githubusercontent.com/assets/1710369/5234903/8197de6c-77e3-11e4-9167-ecc5e53fbcc2.png)

On a side note, I have a question - noticed that some stuff is implemented via explicit modules (i.e. fix usings) and some is generic by retrieving the code actions over the get code actions module. Which is the preferred one and why?