Autoscaling and limits in mplot3d.

    * Fixes improper setting of x,y,z limits not turning off autoscaling
    * Adds units support to z-axis
    * Adds shared axis support for z-axis
    * Closes #783
A bit of cleanup
If the user has "savefig.extension : auto" (deprecated) in their (old) matplotlibrc, it will be copied into "savefig.format", where "auto" is not a supported choice.  This will fix it so it becomes "png" if it's currently set to "auto".  This bug discovered as part of #1530, though it doesn't fix it.
Merge pull request #1537 from mdboom/savefig.extension-fix

Fix savefig.extension == "auto"
Merge branch 'v1.2.x' of github.com:matplotlib/matplotlib into mplot3d/autoscale_fix
Some PEP8 fixes for axes3d.py
Merge pull request #1289 from WeatherGod/mplot3d/autoscale_fix

Autoscaling and limits in mplot3d.
inverting an axis shouldn't affect the autoscaling setting
Renamed all instances of variables named "long" to "lon" to avoid clobbering by 2to3
Remove NO_DEPRECATED flag -- this was accidentally committed.
Renamed all instances of variables named "long" to "lon" to avoid clobbering by 2to3
Remove NO_DEPRECATED flag -- this was accidentally committed.
Added a test for #1553
Remove redundant ob_refcnt's that cause crashes when built in DEBUG mode.
Fixed blitting in Gtk3Agg backend to redraw only area defined by bbox (#1575)
Merge pull request #1578 from spinnau/fix_1575

Fixed blitting in Gtk3Agg backend
Merge pull request #1579 from joferkington/fix-custom_projection_example

Updated custom_projection_example.py to work with v1.2 and newer
Fix #1583: Crash with text.usetex=True and plt.annotate -- usetex bounding boxes are all NaNs when text is empty.
Fix: positional argument specifiers are required by Python 2.6
Fix: positional argument specifiers are required by Python 2.6