Support and tests for adding a reset button to units

@jbau @jrbl  to review please

Decouples reset from randomization while still preserving backward compatibility. Users will be able to effectively clear their input for answere questions that they have not submitted and instructors are able to set course-wide and problem-specific settings.

Implementation is in common/lib/xmodule/xmodule/capa_base.py  
Refactored out common constants in common/lib/xmodule/xmodule/capa_base_constants.py
Unit tests in common/lib/xmodule/xmodule/tests/test_capa_module.py
UI acceptance tests in common/test/acceptance/tests/test_problem_reset.py 
Every new file introduced is a new piece of dereferencing I have to do when I'm trying to read. The ```from import *``` idiom means I have to grep to figure out where these constants come from.

You should specifically document in the file that it is ```from import *``` safe in its doc string. Cf. https://docs.python.org/2/howto/doanddont.html#from-module-import and http://stackoverflow.com/questions/10612798/python-what-is-a-from-import-safe-module . 

But then after doing that, you should additionally stop using the from import * style. One way to avoid it is to package up groups of variables so they're easy to move around together. Since they group so naturally and aren't expected to mutate, you may consider a lightweight object, like:
```
class SHOWANSWER:
    ALWAYS = "always"
    ANSWERED = "answered"
    ....
```
and then you can use them just by changing all your underscores to dots, e.g.:
```
if self.done and self.rerandomize == RANDOMIZATION.ALWAYS:
```
...which would also mean that you could import SHOWANSWER and RANDOMIZATION by name in capa.py:
```from capa_base_constants import RANDOMIZATION, SHOWANSWER```
which makes it nice and clear where these are coming from and how their values are related to one another. 

The all-caps class names are a bit unconventional, but I think the signaling that these are being used as constants is probably more important than the signaling that they're encapsulated as classes, since we're just using classes as containers and not as types in this example.