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
Why are we doing this? Why give the user the option? If we want to give the user the option this doesn't seem like an appropriate way to do it.