Support for multiple coverage files

Updated to support multiple coverage files through command line. Uses CoverageInfo.Join() as mentioned on Peter Huene's MSDN Blog [1].

Preferably `--input` would be a regex pattern, but for now its an array.

[1] http://blogs.msdn.com/b/phuene/archive/2009/12/04/programmatic-code-coverage-data-merging-in-visual-studio-2010.aspx