ENH Add relabel() function

Relabeling does not change the identity of the regions, but renames them
to use 1...K for K objects. This is necessary if some regions have been
obliterated by setting to zero.
ENH Add labeled.remove_regions() function

This is a nice little primitive, so it's good to have it very fast.
DOC Improve docstring documentation
DOC Improve documentation

Folds in the Wally example from stack overflow
DOC Add arXiv citation

This was a FAQ before, but it did not have an answer :)

Now, it does.
DOC Information on contributing

This is all very standard nowadays (github), but it is also standard to
have this explicit in a package documentation.
RFCT Introduce ExtendIgnore mode

This is often very useful.
BUG Fix median_filter: respect the `mode` argument

This was the motivation for the restructuring of the filtering module.
Also, in the ignore mode, this would return the wrong value.
TST Fix comparison with ndimage

ndimage does not have mode
BLD Compile in debug mode from Makefile

The Makefile is mostly for development use and debug mode is often what
we want.

clean & tests are also useful targets.
ENH Add colour conversion module

For conversion between colour spaces.
ENH Add SLIC superpixels function
DOC Mahotas now has over 100 functions

This is the result of ``grep '^def [^_]' | grep -v test_`` which counts
105 functions.
BUG Fix SLIC bug (pass spacer argument through)

Add a demo on SLIC superpixels
DOC New example with fluorescent microscopy
DOC Move documentation sections around

morphology is the weakest: move it to the end
DOC Improve documentation.

Mention improvements to documentation in ChangeLog
BLD A little command to redo all documentation
ENH Early exit from SLIC on convergence

This can be much faster (90ms instead of 310ms on luispedro.jpg with
default parameters).
BLD Add `fast` & `debug` targets to Makefile