Add automated running of notebooks to Travis

Wrote a simple script that makes use of runipy to execute all notebooks from within the current directory and below. Some notebooks take a long time to run, so I'm not sure if Travis will abort our tests or not.

Fixes #20.
Add runipy dependency
See if mpl inline fixes Travis error
Add support for testing specific dirs or a single file

Also fixed some missing imports in the notebooks.
Merge branch 'master' into issue-20
Merge pull request #29 from ElBrogrammer/issue-20

Add automated running of notebooks to Travis