new CubeList method to extract over common coord values
Changed to generator comprehension
Default NetCDF dimensionless unit.
Allow datetime-like values for LinearInterpolator.
Ensure dates-as-nums are floats.
Document the new datetime capability.
Update missing reference in Cube.regrid().
Minor tweak to user guide.
Clarify differentiate() coordinate values.
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.
Add skip_data for tests that require it.
Merge pull request #1263 from QuLogic/tests-skip_data

Add skip_data for tests that require it.
Test normal install with Travis.

Fixes #394.
Merge pull request #1234 from rhattersley/time-linear

Allow datetime-like values for LinearInterpolator.
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.
Search for dot executable in the PATH.

Current code checks if the path exists, but not if it exists in the
user's PATH somewhere. This needlessly requires the site.cfg to be
modified, even though we can run `dot` just fine.
Make tests compatible with Pillow.

Pillow returns Unicode strings instead of str objects, so the tests
fail since there's an extra u prefix in the repr.