Do not restrict limit and offset to a single int

Trying to fix #270 

This is what I could come up with. There probably is a lot of work to do that I just can't see, but at least db functions can be called in limit and offset consistently with other query clauses.

When reviewing bear in mind this is my first elixir pull request and first elixir program in general. Also, please do suggest how to make this more idiomatic, if it isn't asking too much. :-)