python 3 support via 2to3 using setuptools.

Hi,

I'm trying to get biggus to work with python3.  However, I need some help to move forward.

* I'm using the "2to3 approach".  This adds setuptools as a dependency.  I know that some people prefer six, but I think that six is better once the code base **is** python3 and one wants to make it backwards compatible.

* I modified line 1538 from `map(None, shape, keys)` to `list(itertools.izip_longest(shape, keys))`.  I hope that does not impact performance or anything else.

* The only test that fails in my PR is `biggus.tests.test_linear_mosaic.TestLinearMosaic`, more specifically `_cleanup_new_key()`.  It tests the key (at line 560) for int, but my version is returning the key as a numpy.int64 instead.  Any ideas?