Allow pagination to work on _all_docs

[buckett]

This fixes pagination so that it works in situations where the value in the view isn't a String, an example of this is the _all_docs view.

I also updated the guide, but wasn't sure if you wanted the API to contain backwards compatible methods.

This fixes #50

[lightcouch]

Thanks @buckett Fyi, https://github.com/lightcouch/LightCouch/commit/3c43ce85c2fab56ba2724677b3afdf6376d949c3