Phase unwrapping

This PR is an attempt to add a function for phase unwrapping to skimage. First and foremost, does this belong in skimage? I obviously think so.

This implementation is based on a github repository, https://github.com/geggo/phase-unwrap?source=c, and C code from http://www.ljmu.ac.uk/GERI/90225.htm. The licence of the C code is very restrictive, but the following e-mail exchange on a numpy mailing list seems to resolve the licencing issue: http://mail.scipy.org/pipermail/numpy-discussion/2008-November/038888.html. Do we need to do anything more about licencing?

I preserved the history of the previous git repo as best I could when I moved the files into skimage. This means that if you go sufficiently far back in the history, you will have files appearing in weird places. If this is a problem, squashing all these commits should fix the issue (while at the same time losing lots of history).

If I am to proceed with this, the tests need to be looked at and improved, as does the handling of masked arrays.