ENH: scipy.special, Implemented Ellipsoidal harmonic function: the first kind, the second kind and normalization constant 

This is an attempt to implement ellipsoidal harmonic functionality. 
The functions implemented are
ellip_harm, ellip_harm_2, ellip_normal
The sources referred to are:
http://www.geology.osu.edu/~jekeli.1/OSUReports/reports/report_499.pdf
http://arxiv.org/abs/1204.0267
It's best to not use `assert_almost_equal` since it's tolerance defaults are not sensible.

Better to use `assert_allclose(..., atol=0, rtol=1e-8)`.

Also, it's useful to check that the error is of the same order of magnitude as the tail of the sum
```
exact = potential(...)
result, last_term = summation(...)
assert_(abs(result - exact) < 10*abs(last_term))
```