Started renaming "rho" to "alpha"

Got rid of conflicting usage of the "alpha" name.
Code refactoring

- Renamings: task->subject, rho->alpha.
- Fixed a lot of docstrings
- Removed some constructs not compatible with Python 3 (xrange)
- Refactored plot_adhd_covariance2 a lot. Now shorter and much clearer.
- Replaced numpy.linalg.inv by scipy.linalg.inv