Added Selenium page objects for LMS and studio

This is the first step in a transition to using bok-choy for acceptance tests.  A more detailed description of the plan is available here: https://edx-wiki.atlassian.net/wiki/pages/viewpage.action?pageId=44072964

The PR does the following:

1) Moves page objects used for Selenium tests from [the edx-e2e-tests repo](https://github.com/edx/edx-e2e-tests) to edx-platform.

2) Adds a setup.py script to so that the end-to-end tests can pip install the page objects.

3) Adds `bok_choy` to the requirements.

I put the new package in `edx_selenium_pages` in the repo root so it would have a short and recognizable name when pip installed.

The companion PR in the edx-e2e-tests repo is https://github.com/edx/edx-e2e-tests/pull/48

@jzoldak @nedbat