First commit of a polylog code, directly derived from mpmath. Not ready for prime time

hi, this is my first full-fledged hack at scipy, within github etc.... I aim at providing polylogarithm in scipy/special, heavily based on mpmath implementation. The polylog code is not ready at all yet (missing doc and UFunc implementation especially), but this pull request concerns the preliminary hack I needed to do, following a suggestion by Pauli Virtanen, in order to acces directly the zetac CEPHES C function in my cython code special/_polylog.cyx. In order to be able to link against it, I need to extract "special_raise_warning" function from _cephesmodule.c and put it in a separate raise_warning.c file, added to the sources in the sc_cephes.so build. 
Just to give an idea, here is a timeit of the python exposed zetac call, compared to the polylog direct C call :
In [1]: from scipy.special import polylog

In [2]: from scipy.special import zetac

In [3]: %timeit polylog(7,1)
1000000 loops, best of 3: 551 ns per loop

In [4]: %timeit 1+zetac(7)
100000 loops, best of 3: 7.25 us per loop

I would like to have the green like on this preliminary modif to _cephemodule.c : the files concerned thus are 
	new file:   scipy/special/raise_warnings.c
	modified:   scipy/special/\__init\__.py
	modified:   scipy/special/_cephesmodule.c

Johann