Simplified API

This removes the mock.init method and gets rid of the restore return from mock.

To set up a mock file system, call mock().  To restore the original file system, call mock.restore().  To create a mock fs module while leaving the original unmodified, call mock.fs().
Doc updates
Full docs
v2.0.0-rc.1
Merge pull request #1 from tschaub/two

Simplify the API (breaking changes for 2.0).