Implemented MockFile.Open(string, FileMode)

Implemented MockFile.Open(string, FileMode) and related unit tests.

Also, I had to disable and re-enable core.autocrlf for git around my commit, else the entire file showed as having changed. See https://help.github.com/articles/dealing-with-line-endings and http://timclem.wordpress.com/2012/03/01/mind-the-end-of-your-line/.