Updated bok choy page objects setup.py

1) Make setup.py for page objects pip 1.5 installable (use absolute rather than relative paths)
2) Use a more succinct package name (`pages` instead of `edxapp_pages`).  The package is still installed as `edxapp_pages` in the virtualenv.

This will conflict with #2431.  I'll rebase and fix whichever one gets merged second.

@jzoldak