Implemented 'eq' and 'ne' operators as a first draft for Issue 25

Issue 25 requested "is" and "is not" to be able to be used in addition to == and !=. While adding "is" support was simple enough, adding "is not" support is probably much harder (as the not token is already in use, so it's not as simple as adding a new token to the lexer).

As a draft proposal to achieve the same aims as those proposed in #25, while at the same time being lazy, I have added 'eq' and 'ne' operators. Thoughts?