change crate.io to pypi.python.org

Remove crate.io references on README.rst and change to pypi.python.org.

crate.io domain was sold to bigdata company

http://www.reddit.com/r/Python/comments/1wcp93/what_happened_to_crateio/

crateio/crate.io#18

I think at the moment there is no substitute for crate.io.
Are developing a new pypi, which can be seen in [1]

[1] https://github.com/pypa/warehouse