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 not put the check at the module level so it's not done every time the function is called, ie:

```
import scipy import foo
def bar():
    pass
if scipy_version > "0.15":
    foo = bar
```