Added sequence and consumer generators.

sequence is a generator inspired by factory_boy's Sequence class.  It
returns a Python generator which will yield incrementing numbers
forever.

consumer is a perjury generator that enables help you to consume
sequences (or any iterables).  Combined with sequence, you can create a
perjury generator for all sorts of sequential numbers.
Merge pull request #6 from pipermerriam/sequence

Added sequence and consumer generators.