ENH: Connectivity improvements, Phase Slope Index (PSI) estimation

This PR includes:

- Improvement to the connectivity functions to allow combining signals using `zip` (for example). This allows e.g., to combine a seed time series with single trial source estimates, as shown in the example.

- Computation of Phase Slope Index [1]. This will be the first effective connectivity measure in MNE-Python. It uses coherency computed using `spectral_connectivity`, so the implementation is quite simple. The method is illustrated with a new example.

[1] G. Nolte, A. Ziehe, V. Nikulin, A. Schlögl, N. Krämer, T. Brismar, and K.-R. Müller, “Robustly Estimating the Flow Direction of Information in Complex Physical Systems,” Physical Review Letters, vol. 100, no. 23, pp. 1–4, Jun. 2008.

Surprisingly, the results actually make sense ;-)

![snapshot1](https://f.cloud.github.com/assets/647005/341426/601fd66c-9d70-11e2-8860-6540a424d902.png)
![snapshot2](https://f.cloud.github.com/assets/647005/341428/67286942-9d70-11e2-8f99-bbc006c4d9d8.png)