Limit and skip support

I needed limit and skip support for my application - I don't know if this is useful to you, but I figured I'd offer it back ;)

I also created two empty signatures, which just enable you to call the JS without including limit or skip of you want.

It didn't look like plv8 liked having skipped params, but you can define a function with multiple potential options.

Anyway, if this isn't useful to your usecase, feel free to ignore, but this makes porting my app from Mongo->Postgres easier.

-CPD