[MRG]: add fast_dot function calling BLAS directly and consume only twice the memory of your data

Hi there, I finally got it running.

This implements a feature 'advocated' on this scipy page (section on large arrays and linalg):

http://wiki.scipy.org/PerformanceTips

When directly calling blass instead of np.dot it's possible to avoid copying when data are passed in F-contiguous order. In addition I've added chunking to the `_logcosh` function which avoids an extra copy.
This is now how it looks on 1GB testing data:

![fast_dot_chunking_logcosh](https://f.cloud.github.com/assets/1908618/861320/cb8e7c78-f5cc-11e2-8e1e-59727b66f4b2.png)

This was how the same test would have looked on the current master (plot from the last memory PR):
![memory_ica_par_w1_computation_del_gwx](https://f.cloud.github.com/assets/1908618/861329/f0928b36-f5cc-11e2-82b6-01d17d79190a.png)
 
To make this functionality available for other use cases I've added a `fast_dot` function to `utils.extmath` with almost stupid but explicit tests that exemplify the mapping between `np.dot` and `fast_dot` which can be a hell.
Finally I've made sure that down-stream applications are still workin. For example with this local branch the mne-python ICA looks as good as it had looked before.

cc @agramfort @GaelVaroquaux @mblondel