Math functions for long ints

Here they are!

There's still a problem when running the [prime_decomposition](https://github.com/coyotte508/pythran/blob/master/pythran/tests/rosetta/prime_decomposition.py)  code on 32 bits (aka by using long ints): 

in the `fac` function, the recursive call to `fac(n//q)` calls a `fac` function with a different type (gmp_binary_expr(...)) than the original gmp_expr and subsequent recursive calls continue to create new fac functions and expand the type, until g++ hits the template recursive depth limit: http://pastebin.com/xKiKDvSA.

I also noted a minor problem with the `factorial` function in math: Pythran's will work fine if provided with `2.5`, but in python code this would raise an exception since it should only take integers or floats with a null decimal part as arguments.

There's also two tests commented out in test_long.py because they don't work, i'm not sure why.

Unrelated, but I started a [wiki](https://github.com/coyotte508/pythran/wiki), where I'm writing a developer guide, with info that would have helped me a lot with the changes I'm making to pythran. (Writing new stuff as I understand new stuff)

It's in its infancy, but maybe when it's more complete I can format it properly and make a pull request to put it in the doc's devguide :o