Add a built in Console reporter to support CI systems

[talarari]

I'm running my tests on CI machines that don't have a diff program installed. It seems reasonable to me that this library would include a built in diff reporter so it doesn't have to depend on other programs being installed on the machine.

I bet there are decent diff nugets out there, its possible to just use on of them.

Great work!

[talarari]

made this PR: https://github.com/droyad/Assent/pull/14 Didn't know how to test the reporter (do you use a sample app or something?)