Overhaul of installation

This PR includes a number of changes designed to make installation easier: these include

* Switching from distutils to setuptools, which better handles making executable scripts on windows
* Creating the `glue_deps` command line utility, to manage Glue's dependencies
* Coaxing ATpy into using astropy libraries for vo and asciitable, if available (effectively eliminating these two dependencies, but still supporting them at the moment)
* Updating the installation documentation, recommending the use of Anaconda

This hopefully makes installing glue a stable, ~3 step process
1) Install anaconda, to grab all the non-trivial dependencies
2) Install glue
3) Run glue-deps to install trivial (pure-python) dependencies

The next step is to deprecate the ATpy dependency in favor of an astropy-only approach to table loading. @astrofrog Is working on this