Porting code to be compatible with py3 and py2

Hey, folks!

I ported the code to work also for Python 3 (specifically, 3.3 and 3.4).

Summary of the changes:

* using absolute imports instead of relative
* adjusted tox.ini and .travis.yml to test also with py3
* adapted new_relativedelta overriding ``__init__`` instead of ``__new__``
* using dict.items and dict.values instead of dict.iteritems and dict.itervalues
* using ``next(some_iter)`` instead of ``some_iter.next()``
* protected code dependant on specific versions (ideally, this would go away)

I'm also commenting below some stuff I'm not sure if it's handled the best way.

@csalazar @Allactaga Any concerns?
+1 to using unicode when possible - this line looks suspicious. But it could be the case 2.x stdlib doesn't handle unicode properly; it is better to check that. When checking that make sure C locale is set, something like this: https://github.com/kmike/DAWG/blob/ad20bafc99218ee045d7face1ec6155b3b276400/tox.ini#L13.