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
delaunay linear_interpolate_grid: enable grid of height/width 1
fixes #997
PEP8 fixes on dates.py
delaunay: test for 1d and 0d grids (fails upstream) re: issue #997
Merge branch 'iss997_test' into iss997_altfix
check `ret is False` in Timer._on_timer

The docstring states:

> Functions can return False if they should not be called any more.

Cleanup commit 2f11dee changed this logic,
so any return value whose boolean interpretation is False (e.g. None)
would be unregistered.

fixes #1492
add comment to protect against future 'cleanup' of `if ret is False`
test_delaunay: avoid numbers in 1d test, use np.allclose
Merge branch 'iss997_test'
_delaunay.cpp: support [x0:x1:1j] grids even if x1!=x0
use ret == False instead of ret is False

per review
BUG: Mask NaNs in color arrays passed to streamplot

Bug report and fix provided by Jon Ramsey.
TST: Add tests for streamplot
set_cmap should not require an active image

Before this commit, it was not possible to change the default colormap if there
wasn't an active colorable artist, i.e. if plt.gci() returned None.

There's no reason why we should force this, though, since part of what
plt.set_cmap does is setting the default colormap for all FUTURE colorable
artists which will be created.

Colormaps which are currently exposed as their own functions in pyplot, such as
plt.gray(), plt.hsv(), plt.hot() and plt.jet() make no such restriction, they
do not complain if plt.gci() happens to return None.

Prior to this commit
```
In [2]: plt.set_cmap('bwr')
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-2-fa42f8679abe> in <module>()
----> 1 plt.set_cmap('bwr')

/home/pi/.local/lib/python2.7/site-packages/matplotlib/pyplot.pyc in set_cmap(cmap)
   2029         im.set_cmap(cmap)
   2030     else:
-> 2031         raise RuntimeError('You must first define an image, eg with imshow')
   2032
   2033     draw_if_interactive()

RuntimeError: You must first define an image, eg with imshow

In [3]:
```

After this commit, no error is thrown, and the appropriate colormap is activated.
```
In [2]: plt.set_cmap('bwr')

In [3]:
```
pyplot.plotfile: forced grid draw removed and documented
PEP8-compliance on axes.py (patch 3 / 4)
Remove unused baseline images
DOC: github ribbon does not cover up index link

thanks @jenshnielsen for spotting the fact that the previous "solution"
didn't actually work as intended.
pgf: documentation enhancements