Create dedicated offset method.

 - allows offset to be specified separately from limit
 - becomes a little bit more simply compatible with Arel API. :(
Minor additions to Dataset#offset support

Update documentation to reflect the method.

Make Dataset#limit call Dataset#offset if adding an offset.

Add offset to the query methods so a mutation/model dataset
method are created for it.

Add some specs regarding interaction of the offset/limit methods.
Merge pull request #1 from jeremyevans/master

Update the Dataset#offset support