#45: Make it possible to override the backend
#45: Make it possible to override the backend

By setting the environment variable BACKEND, we can now
choose a different backend. Ideal for testing it on the box
itself, which is a lot faster. Example:

BACKEND=exec rake spec

joomlatools/joomlatools-vagrant,joomlatools/joomlatools-vagrant,joomlatools/joomlatools-vagrant,joomlatools/joomla-vagrant,joomlatools/joomlatools-vagrant,joomlatools/joomla-vagrant,joomlatools/joomla-vagrant,joomlatools/joomla-vagrant,joomlatools/joomla-vagrant,joomlatools/joomlatools-vagrant,joomlatools/joomla-vagrant