Add support for conda package manager

The [conda](http://conda.pydata.org/docs/) package manager is gaining popularity, mostly in the scientific Python community. It allows installation of binary packages in separate environments, similar to Python virtualenvs, but not limited to Python packages.

The tools added in these commits (fabtools.conda, fabtools.require.conda) allow the installation of miniconda (a minimal conda environment) as well as  setting up conda environments, package installation etc., much like the fabtools.python and fabtools.require.python packages