Add support for custom evaluators using compiled langauges

[Holmes98]

This adds an optional language field to the Evaluator class. All existing/new evaluators will default to having no language, which maintains the existing functionality (script is executed as-is).

Note that C++ evaluators were already "supported" through cint.rb, but that is much slower as it requires the evaluator to be recompiled on every test case. That functionality can be considered deprecated and removed later, after fixing existing evaluators.

Also note that for evaluators with no language, Pygments will attempt to automatically detect the language based on the code (see here).

[coveralls]

coverage: 37.481% (-0.01%) from 37.494%
when pulling 8d84fcd on compiled-evaluators
into 4fc9595 on master.

[BelgianSalamander]

I don't know too much ruby (or rails) and am not too familiar with the codebase but the code changes seem to make sense.

I tested it locally for a bit and it worked as expected but I'm not sure about raising an error when evaluator compilation fails.

[BelgianSalamander]

This is probably outside the scope of this pr but in future it may be good to not display the message This submission has not finished judging. Refresh this page in a minute or two to see the submission's score. when judging encounters an error (be it evaluator not compiling or some other error in the worker).

[Holmes98]

Yeah, the submission page doesn't really handle judging errors. I've committed some improvements in 9e7aa3e.

[Holmes98]

CI is currently failing; need to merge #266 first.

[Holmes98]

Ugh, now coveralls is failing...

[BelgianSalamander]

Seems to be fixed now

[Holmes98]

Deployed and converted all existing C++ evaluators that were previously using cint.rb to use this instead.