fixes issue #2556

Fixes it in two ways:

1) changes the closure of `on_dpi` to be over a weakref, not `self`
2) cleans up the call back when the quiver is removed
pep8 fix
Don't snap pcolor
renamed import

renamed imported matplotlib.collections from
`collections` -> `mcollections` to try and avoid clashing with
`collections` in python core.
added test_quiver module

tests memory leaks reported in issue #2556
fixed memory leak in quiverkey

closes #2556
pep8 + minor variable re-naming
removed un-needed weakref
added missing self
Added missing warnings import

There is a warnings call around line 732, but as warnings is not imported, this fails.
Merge pull request #2757 from iMichka/v1.3.x

Added missing warnings import
TST : test `plot(.., color='none')`

added failing test for issue #2760
BUG : make `plot(..., color='none')` work again

Fixes #2760

Fixes miss-handling of color conversion logic introduced in edc48f0
BUG : makes Axes.margins work with just kwargs

applied fix supplied by @syoung-m4 in #2510
Merge pull request #2762 from tacaswell/margin_arg_counting

BUG : makes Axes.margins work with just kwargs
Merge pull request #2705 from mdboom/fix-numpy-1.9

Build fails on OS X with NumPy 1.9
Conflicts:
	src/file_compat.h
	src/ft2font.cpp
Merge pull request #2558 from tacaswell/quiver_memory_leak

fixes issue #2556
Merge pull request #2761 from tacaswell/fix_line_color_handling

Fix line color handling
Make it possible to control snapping from the outside.
Merge pull request #2726 from mdboom/pcolorsnap

Don't snap pcolor