ADD Generate Test Coverage Report for the API server

[PIG208]

This allows us to run tests/test.sh to generate a test coverage report using coverage.py with pytest.

The report can be generated with make coverage under api.

An example of the report:

Name                                        Stmts   Miss  Cover   Missing
-------------------------------------------------------------------------
anubis/__init__.py                              0      0   100%
anubis/app.py                                  32     28    12%   11-22, 34-52, 64-77
anubis/views/super/__init__.py                  7      7     0%   1-13
anubis/views/super/config.py                   29     29     0%   1-73
anubis/views/super/ide.py                      64     64     0%   1-114
anubis/views/super/playgrounds.py              37     37     0%   1-88

......

jobs/autograde_recalculate.py                  21     21     0%   1-60
jobs/bot.py                                   106    106     0%   1-307
jobs/daily_cleanup.py                          36     36     0%   1-112
jobs/poller.py                                 16     16     0%   1-40
jobs/reaper.py                                 55     55     0%   1-167
jobs/reporter.py                                7      7     0%   1-15
jobs/visuals.py                                17     17     0%   1-34
-------------------------------------------------------------------------
TOTAL                                        5073   4503    11%

[wabscale]

Good stuff