Update Axes3D.tricontour for custom triangulations

An example has also been added to illustrate the additional flexibility
possible with providing a user-defined triangulation.
Make Axes3D.contourf take arbitrary triangulations
Add Axes3D.contourf example
Update changelog/whats_new for 3D tricontour plots
Typos
Add EXPERIMENTAL note to Axes3D.tricontour[f] docs
Fix compilation on Solaris
color of candlestick lines

the line of a candlestick should have the same color as the body (rectangle)
Update of mlab.pca - updated docstring, added saving the eigenvalues.

The old version of pca class did not save the Eigenvalues - just the "fractions" that are just percentage values.

Also added info which row of Wt one can find the Eigenvectors.
DOC:Use monospace for --

The -- is rendering as an en dash on http://matplotlib.org/users/pyplot_tutorial.html

Maybe all strings should use monospace instead of single quotes?
axes_grid1: ImageGrid respect the aspect ratio of axes. This addresses #955
DOC: Change all strings to ``'monospace'`` in quotes
DOC: Change strings to monospace
make all optional backends respect the cfg file
make sure we only perform absolute imports on loading a backend

Fixes #2309
cleaning up OptionalBackendPackage a tiny bit

 - check for `if config is not None` before the try/except/pass adds
   nothing, just makes it less readable
 - much clearer to handle of the default value of the `install` variable
   with the base class and not nested in a try/except of the
   `get_config` method (and even duplicated outside of the try/except)
 - the `force` attribute (only used for BackendAgg right now it seems)
   should also have a more prominent default state in the base class
   rather than in just one subclass
move general check() method of optional backend to base class

also split up the if/else in 3 branches to show a separate message if an
configuration file backend opt-out was overridden by a forced backend
install
replace checking of configuration of optional backends in overridden check() methods by a super() call
refactoring/separating backend checks for config file/dependencies

 - config check is handled handled in optional backend base class and
   calls separate dependency check
 - dependency check is an empty stub in base class and can be overriden
   in subclasses
update list of gui backends in config template