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?
My problem with this is that the ``date_string.encode('utf-8')`` is reverting back from Unicode, using an encoded byte string instead of the unicode string.
Can't we use the unicode string in this method just like we do it elsewhere on the code? (DateParser, FreshnessDateParser)