Fixed unusedArg comparison.

Though the unusedArg should only be strings, if it were not to be,
comparing to undefined is more accurate than checking for truthiness.
Add keyword.evaluate test.
Add keyword.use test.

Also refactor the Shell spy out into beforeEach.
Remove unused args from keyword.use.
Add keyword.it test.
Add unimplemented keyword methods.
Merge pull request #121 from mcomella/test_keyword

Add keyword module tests