Demote dependencies & configurationize Selenium

* Enable configuration of Selenium webdriver via environment variables
  to specify Chrome or Firefox (and hopefully in the future other
  things)
* Enable conditional imports of non-critical, non-unit-tested libraries
  so that their installation can be avoided when unnecessary.
* Creates suggested_requirements.txt to document which libraries these
  are and give users a chance to pip install -r them all.
* Update documentation to reflect requirements.txt and
  suggested_requirements.txt as the final source of truth for
  Python dependencies.
* Pin gdata version at 2.0.17 as the comments in README_SETUP.md
  previously indicated it should be. Pin selenium version >= 2.30 so it
  works with recent firefoxes.
* Configure Travis to use Firefox with the Adobe Flash plugin installed
  for Selenium testing. This fixes Selenium testing on the grid.
  (closes #2272)