MAINT: reorganize source tree to something more sane.
BLD: update setup.py
MAINT: move _pywt.pyx to pywt/src/
MAINT: add pywt/setup.py, scipy style.
BLD: more fixes to make new repo layout work.
BLD: tools/six.py was still not being installed, fix.
MAINT: remove util/commands.py
TST: update tox.ini, use virtualenv outside source tree.

Reason: then it doesn't get cleaned up with ``git clean -xdf``.
Merge branch 'reorganize' into master.

Reshuffles the source tree to a more standard layout.
Both normal installs and in-place builds work now.

Closes gh-28 and gh-13.