Include tests_require argument in setup.py
Include mock as test dependency

This is only needed for python 2 because mock is in stdlib for python 3
(unittest.mock).
Add base and NDArray backend

This commit contains a new backend for sampling and selecting values.
Non-backend files have been changed to work with the new backend.

`sample` and `psample` functions have been merged. `sample` now takes a
keyword argument `njobs`, and if this is over one, the multiprocessing
version is used.
Add Text backend
Add SQLite backend
Add backend documentation
Add shortcuts for Text and SQLite backends
Add SQLite backend example for hierarchical.py
Merge pull request #527 from pymc-devs/pymc3-backends-3

Text and SQLite backends (update)