"Edit page on github" reformat

This is a slight modification of the improvements from #617.  It just changes the case convention/formatting of the page footer in the docs to be consistent.  Instead of "[edit this page on github]", "Page Source", and "Back to top", it is now "Edit This Page on Github", "Page Source", and "Back to Top". Compare http://docs.astropy.org/en/latest/ to http://eteq.github.com/astropy/ (the footer of the index page shows the change).

@mdboom - is this alright with you as the implementer in #617?