Recommend using virtualenv instead of setup.py develop with py.test

Issue #424 was fundamentally caused by using python setup.py develop. There was a .pth file with a path pointing back to the git repo, so when trying to import astropy it was always picking up files in my git repo source directory instead of the installed version.

This PR updates the testing guidelines to recommend using virtualenv instead of develop for using py.test.

This is a re-work of PR #426 based on comments from @mdboom.