Issue 149 check vcs existence

* Fixes #149 
* When user attempts to use `cookiecutter` and calls on `git` or `hg` without them being installed, `cookiecutter` returns a nicely explicit `VCSNotInstalled` exception.
* Added `test_compat.py` to test against new `which` function.
* Documented the compat module in `docs/cookiecutter.rst`