Updated the 'Installing Astropy in develop mode' section ...

... (use setup.py develop in the first instance, added a link to the instructions on how to run the tests, which required adding a label to install.rst), added some additional links to GitHub pages, fixed some typos, applied other minor edits for clarity / consistency.

This is my first pull request for astropy (I'm at the Scipy2013 sprints!), so let me know if I need to change anything :)