Python 3 Support

This changes based on [Curita:pytest](https://github.com/Curita/scrapy/tree/pytest) branch.
Now runtests with py.test for python 2.7 discover 980 items / 12 skipped.
I made a few changes for add compatibility with py3k, replacing imports with six-module.
Now runtests with py.test for python 3.4 discover 678 items instead of [52](https://travis-ci.org/scrapy/scrapy/jobs/22315206#L53).

I plan to continue improving of compatibility coming soon.