Refactor rake tasks for acceptance tests
Address PR comments

CMS tests should now run on IE

CMS tests will now run on Safari

Rebased wrong

Forgot to update the test_acceptance.sh

Changed way of string splitting

Changed to have a default for device since it isn't always needed

Tags are more clear

Fixed stylistic issues

This logic also covers if a lettuce_browser is specified

Removing browser matrix from readme

test suite, so the clean reports command doesn't blow
away the reports.

Original change: SHA 58e98d
This was resolved by merging in Cale's branch
  https://github.com/edx/edx-platform/pull/44

Do a bundle install before trying to use rake, so that we have all the prerequisites when running tests

We used to specify specific rake test tasks so that we could run all of
them even if early ones failed. However, that meant that as new tasks
were added, they weren't being run on jenkins.

Now, there is a facility in the rake scripts so that tests can run using
the test_sh function, which will delay failure until the end of the rake
run, unless the TESTS_FAIL_FAST environment variable is set.

Furthermore, this reorganizes the jasmine test tasks so that we can run
those as part of `rake test` as well.

Added diff coverage reports using diff-coverage tool.

Fixed a typo in the original jenkins/test.sh script

Make sure that test-requirements.txt doesn't install versions of libraries that requirements.txt has pinned, and cache pip downloads