Make it possible to run utilities tests with --watch

[hazeledmands]

Hey! I'm starting to play around with possibilities for an overwriteChainableMethod utility to address issue #215, and I was trying to run the utilities tests with:

mocha --watch --require ./test/bootstrap --reporter spec test/utilities.js

But the tests fail if they run more than once because they expect certain methods to not be part of the Assertion prototype. Would it be okay for me to add some cleanup in an after() block?

[coveralls]

Coverage remained the same when pulling e7cf4d4 on demands:test_cleanup into 1654808 on chaijs:master.

[hazeledmands]

...updated my commit message to match the others, I think.

[coveralls]

Coverage remained the same when pulling 4ce3d42 on demands:test_cleanup into 1654808 on chaijs:master.