DOC: Clarify UMFPACK warning; use if statements instead of assert
FIX: Shortcut output or catch trivial cases in random_walker

Also:
* New tests to cover these new checks
* Improvements to docstrings and user warnings
* Generalize handling of `sampling` in accordance with docstring
* Some extra whitespace to improve readability
TST: Add unit test for length-2 spacing with rank-2 input
Replace broken _import_tools from numpy with Git version
Print versions of software used in Travis output
Remove deprecated --use-mirrors from pip calls
Rename heading -> header
Don't fail if .matplotlib dir already exists
Merge pull request #997 from stefanv/build/travis_numpy_py3

Fix Travis-CI builds with numpy 1.8 under Python 3
Fix cython compilation warnings
Fix custom six issue in scipy 0.14 on python 3.4
Merge pull request #998 from ahojnnes/python34

Fix cython warnings and "six" under python 3.4
STY: More elegant and maintainable style per code review
Remove check for non-numeric values in spacing.
Merge pull request #984 from JDWarner/randwalk_trivialcheck

Random Walker improvements and checks for trivial cases
FIX: Remove unnecessary .transpose introduced in PR #984
Merge pull request #1001 from JDWarner/rw_transpose

FIX: Remove unnecessary .transpose introduced in PR #984
failUnless -> assertTrue
Merge pull request #1002 from JDWarner/asserttrue

Address PendingDeprecationWarnings in watershed tests
Added files and required code for data images gallery.