ENH: add axis keyword to dendrogram function

This fixes gh-1325 (Trac #798). It allows scipy dendrograms to easily be
plotted on custom axes, e.g. as part of a larger figure. It does not import
pylab at all if an axis is provided, which is useful for those who are using
the matplotlib API.
MAINT: changed 'axis' to 'ax' throughout dendrogram code.

Also changed "axis_gca" flag to be more descriptive "trigger_redraw".
TST: added test for dendrogram plotting

Tests plotting to pylab.gca() and to an axis passed in as an argument.
DOC: added documentation for the ax keyword of dendrogram().
DOC: fix up formatting of cluster.dendrogram docstring.
TST: clean up cluster.hierarchy tests for docstrings.
Merge branch 'pr/3201' into master.

Reviewed as https://github.com/scipy/scipy/pull/3201