Optimized version for polar shaplets

Hello Griffin,
first of all thanks a lot for implementing this shapelets package!
A year ago or so I wanted to use it to model (and remove) background noise in some experimental images. This  pull request contains some optimisations for polar shaplets:
  -  Cython version of the  genPolarBasisMatrix: requres the GSL library and the cython_gsl module
  -  rewritten pure python version so it can be used with numexpr: it turns out that the numexpr version is actually faster and easier to setup. 

For instance for a problem size of  (1000, 500),  the execution time on my laptop is 
  - original : 17.4 s
  - cython : 4.25 s
  - numexpr : 3.49 s

The `script/profile.py` file contains the code to run this test and lists the used parameters.

I include both versions here  if ever it is of  interest to you, but otherwise I can do another pull request with only the numexpr version as it is easier to maintain and setup. Curretly the 3 implementations can be used for the genPolarBasisMatrix function
```python
import shapelets.decomp as sh
import shapelets.fshapelet as fsh
import shapelets.cshapelet as csh
# original version
sh.genPolarBasisMatrix(*args)
# numexpr version
fsh.genPolarBasisMatrix(*args)
# Cython version
csh.genPolarBasisMatrix(*args)
```
There are some unitests that validate that all of the implementations give the same results:
```
   $ cd src/ ; nose -s tests.py
```

Best,