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?
So, I dug a little deeper on the commit logs, and I found out that this was added because of this sanitization: https://github.com/scrapinghub/dateparser/blob/master/dateparser/date.py#L71

Now, I've checked that this sanitization works with unicode strings too, in both Python 2 and Python 3:

```
$ python
Python 2.7.6 (default, Mar 22 2014, 22:59:56) 
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from dateparser.date import sanitize_date
>>> sanitize_date(u'\t\n\r\u00bb\xe0, \u0432\u0433.\u200e\xb7') == u' '
True
```

```
$ python
Python 3.4.0 (default, Apr 11 2014, 13:05:11) 
[GCC 4.8.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from dateparser.date import sanitize_date
>>> sanitize_date(u'\t\n\r\u00bb\xe0, \u0432\u0433.\u200e\xb7') == u' '
True
```

Therefore, I think it's safe to remove this. Any thoughts?