OSPR-535 Partial Credit

Squashed commit of the following:

commit 0f7c2af5f7b8caed575dd253a45299293b2729d7
Author: Colin-Fredericks <colin.fredericks@gmail.com>
Date:   Tue Jun 30 12:04:43 2015 -0400

    Forgot icon

commit b48970392741130f774709c54eb6e5ab0089812c
Author: Colin-Fredericks <colin.fredericks@gmail.com>
Date:   Tue Jun 30 11:49:57 2015 -0400

    OSPR-535 Partial Credit

    Squashed commit of the following:

    commit 6dd34f58f994e32d0d54bf1d67bffd04e0f8ef08
    Author: Colin-Fredericks <cof945@dhcp-140-247-184-176.fas.harvard.edu>
    Date:   Tue Jun 30 11:44:01 2015 -0400

        Fixing accidental overwrite.

    commit 1ff8fc4b0e83b90356e8e8dce1022f49bfd162cf
    Author: Colin-Fredericks <cof945@dhcp-140-247-184-176.fas.harvard.edu>
    Date:   Tue Jun 30 11:18:36 2015 -0400

        OSPR-535 Partial Credit

        Revised after first pull discussion.

Fixing scss typos

Fixing check/x display problem

Empty set is not []

Shuffling empty answer code to grade properly.

I don't think I ever wrote this in the first place...

Adding tests for MC and Checkbox

including proper partial-credit marking and scoring

Numerical and OptionResponse tests

Also a few improvements to NumericalResponse problem type and
exception-raising.

CustomResponse tests and more numerical tests

Increasing coverage and fixing typos

Exception added for pylint false positive

Hopefully fixing coverage issue

Retabulating line continuation

Bok Choy test for partial credit

Copypasta fix

Adding tooltip for partial credit

Improving and expanding comments

Minor fixes