SA1300 ElementMustBeginWithUpperCaseLetter - Implement Unit tests

Fixes #177, Contains #3.

I rebased the pull request by @sharwell onto master and implemented unit tests. Currently the tests for fields are failing because the diagnostic creates two warnings for them. Roslyn seams to call the diagnostic twice and I don't know why just now.

I also have written a CodeFix for this, but I need to add some more tests before I create a pull request.