tox - testing using tox, fixed reading version for setup.py, cleaned requirements*.txt

Fixed bug in setup.py, which was importing arrow before required python-dateutil was installed.

Added tox.ini to allow testing under various Python versions.

Added testing for Python 3.4

Removed Makefile (tox made it obsolete).

Updated .travis.yml to use tox

MANIFEST.in now includes for sdist all files needed for testing ($ devpi test arrow is passing now).
I see, you want also `VERSION` to be set up. I will check, if it is used somewhere and add it.

Are you aware of using `VERSION`?

In Python packages it is usual to have just `__version__` sometime being available via functions `version()` and `version_info()`.

`VERSION` sounds more like from C world.

I will add it, but if you think, we can live without it, I would stop using it.