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.
Add pp_packing to list of pep8 exclusions.

Since it's a generated file, there's not much chance of us fixing it.
This file is not always created though, so the exclusion needs to be
made optional.
Change config creation so that udunits loads.
Write ungzipped files to a temporary path if needed.

This may occur if the original test data are installed in the system,
and consequently, the ungzipped version cannot be placed in the same
directory.
Merge pull request #1249 from QuLogic/travis-test-user-install

Test user install on Travis