make sure we only perform absolute imports on loading a backend

Fixes #2309
Add a comment explaning __import__ syntax.
Merge pull request #2358 from mdboom/fix-inline-figures

Broken IPython notebook integration
Merge remote-tracking branch 'upstream/v1.3.x'

Conflicts:
	.travis.yml
	lib/matplotlib/backends/qt4_editor/formlayout.py
Better axis limits when using shared axes and empty subplots
Merge pull request #2357 from ChrisBeaumont/empty-shared

Better axis limits when using shared axes and empty subplots
[bug correction] triinterpolate is independant of triangulation numbering.
Merge pull request #2363 from GBillotey/new_test_triinterpolate

[bug correction] trirefine is now independant of triangulation numbering
Merge pull request #2335 from andreabedini/patch-5

make sure we only perform absolute imports on loading a backend
Use six instead of 2to3
Add document with tips for writing portable 2-3 code