Word cloud tests

New additional tests for Word Cloud: integration tests, mongo tests, pylint+pep8+coverage fixes.

Reports:
------------
**tests + coverage**
```bash
$ coverage run `which django-admin.py` test --traceback --settings=lms.envs.test --pythonpath=. --logging-clear-handlers $(git diff master --name-only | grep -i '\.py$')
.............
-----------------------------------------------------------------------------
13 tests run in 11.8 seconds (13 tests passed)

$ coverage report -m $(git diff master --name-only | grep -i '\.py$') common/lib/xmodule/xmodule/word_cloud_module.py
Name                                                  Stmts   Miss  Cover   Missing
-----------------------------------------------------------------------------------
common/lib/xmodule/xmodule/tests/test_logic              58      0   100%
common/lib/xmodule/xmodule/word_cloud_module             68      0   100%
lms/djangoapps/courseware/tests/__init__                 38      0   100%
lms/djangoapps/courseware/tests/test_word_cloud          62      0   100%
lms/djangoapps/courseware/tests/test_word_cloud_xml      22      0   100%
-----------------------------------------------------------------------------------
TOTAL                                                   248      0   100%
```

**pylint**
```bash
Your code has been rated at 10.00/10 (previous run: 10.00/10)
```