Feature: Cram plugin

Cram is a functional testing framework for command line applications. This pull-requests presents a pybuilder plugin for it.

Cram and the details can be found on: https://pypi.python.org/pypi/cram

There are also two dog-fooding test files in `src/cmdlinetest` which is the default location for cram tests. The location can be configured, as can the glob used for discovering cram tests. The plugin respects the `verbose` setting.

One thing I wasn't able to work out, was how to launch the plugin as part of `publish`, so remains as its own task for now. 

Another issue is that the unit test is not exactly DRY right now.

Anyway, I wanted to submit this for review now, because it is already in a working state and just needs some more polishing.