Modified monkeypatching code

Modified so it only defines String.prototype.plural if it currently
does not exist.

Also removed some unused variables.
Switch monkeyPatch/unmonkeyPatch tests

Just switching tests around
Tests now do not use plural that modifies natives

Instead of using the function that extends String's native object,
we will just use the provided plural function.

Also changed the example to just use plural() rather than using String.
Added VERSION property

VERSION references the version from package.json