Move TestRunner class into iris.tests.

This allows one to easily test the installed package by using the
command 'python -m iris.tests.runner'.
Add configurable documentation path for examples.

When installed, the documentation is probably not going to be exactly
alongside the test script. It may even differ between different systems,
so this config option allows the installer to specify its location
similar to the test data.
Test normal install with Travis.

Fixes #394.