ENH: Add plot_trans function

Plotting of -trans.fif files
![screen shot 2014-06-08 at 11 05 06](https://cloud.githubusercontent.com/assets/161052/3210862/44920856-eeec-11e3-8379-3977f58adbbd.png)
![screen shot 2014-06-08 at 11 04 53](https://cloud.githubusercontent.com/assets/161052/3210863/44a3ed14-eeec-11e3-95ad-f1cc879acd30.png)
![screen shot 2014-06-08 at 11 04 41](https://cloud.githubusercontent.com/assets/161052/3210864/44b06fe4-eeec-11e3-99ea-2054ecbc69e0.png)

see examples produced with mne_analyze

one could also show the dig points. Would be good but you can keep this for another PR