ENH: DataFrame.corr(method='spearman') is cythonized.
It should be more than 10 times faster than the old version.

ok....can you put a release notes mention? (enhancement section),

and if you are gung ho, pls add...a vbench entry (vb_suite/stat_ops.py)....it could have a start date of say 2013/1/1 (or earlier if you can figure out about when spearman was added....was sometime last year, but I don't remember)
 @jreback is there any consensus about fused types/memoryviews? if there's a consensus to use should we "require" that new cython use it?
 should use them
but in this case doesn't matter as float64 is all that is needed
 fused types sure but what about memoryviews here?
 if u can get them to work
should do it
 @jniznan does @cpcloud change make sense?
 Yes, I will push the changes in the next day or two.
 @jniznan can you see if you can finish this up ? thanks
 @jreback  sorry about the delay, 
it seems that the spearman functionality was added on 2011/12/3 (GH #428) , so i will put 2011/12/4 in the benchmark. 
 @jreback i put in the release notes mention and the vbench entry, i have also implemented the change @cpcloud proposed
 @jniznan can u rebase?
 @cpcloud it should be rebased now.
 Merged...thanks.....nice improvements

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
stats_corr_spearman                          |  83.9807 | 3735.8600 |   0.0225 |

Target [a70b48a] : PERF: stats_corr a smaller vbsuite
Base   [c4a218c] : Merge pull request #3929 from y-p/PR_PTF_no_more
```
