[tests] Change example module tests

Add to and improve the quality of the tests for the example module. This
commit also fixes a small bug in the example module when dealing with
sparse masks to the example.rand function.
[tests] Use nosetests to run doctests on travis

Add the --with-doctest flag to the nosetests that are run on Travis CI.
Remove the calls to doctest if the __main__ of each modules. This
closes #1.
[import] Add underscore before imported modules

Import modules with an underscore in front of their names. For example
import numpy as _np. This is to address the namespace getting to full.