Fix layer backdrop getter tests

Fix failing doctest for the layer backdrop determination method. Switching
to an isolated "normal" mode for layer groups by default in c413a96 broke
the test.
Update lib/ code to allow headless testing

Nose uses a slightly different environment from the one I've been testing
under at my end. Removed an extraneous Gdk import as well.
Test build in Travis

Only the non-GUI parts, though - the Travis build farm boxes are likely to
be headless boxes.
Reformat & update README

Reformat README.md for friendlier diffs. Hopefully it'll still render OK.
Add notes about running the unit tests via nose.

[ci skip]