Added documents bulk retrieval

[doubiz]

Added 2 methods called get_bulk and `get_bulk!' that retrieve multiple documents given an array of ids.

[samlown]

@doubiz thanks for looking at this. Is there any specific advantage to your approach compared to just running WithDefaultValues.all.keys(['id1', 'id2', 'id3'])? I'm guessing the fact that you don't need a Design Document, but I'd just like to confirm this is your use case.

In addition, I'd like to ask for a slight reversal of the logic. I'd suggest that get_bulk is the main method, and get_bulk! raises the exception if get_bulk provides an empty response. I'd also suggest sticking with the default singular error message as opposed to a plural version to avoid confusion. Do you agree?