Faster spherical harmonics

Thanx @ChantalTax, @pv and @stefanv for helping with this. Here is a faster implementation of spherical harmonics. This should make the calculation of real spherical harmonics and complex spherical harmonics faster everywhere in dipy. 

Let me know what you think! This is now 100X faster. :D
I used what it was in the documentation of scipy.special.sph_harm
http://docs.scipy.org/doc/scipy-0.14.0/reference/generated/scipy.special.sph_harm.html

I think it is fine to say broadcasted. Even if it is not in some dictionaries. It is commonly used.