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?
I am a bit concerned about this constants defined throughout code. Maybe it is better to create a `portability` module, and define all required functions and constants there in one place? Or better yet, use [six](http://pythonhosted.org/six/) :smiley: