Added high_variance_confounds() and tests
Coding-style corrections

- Renamed parameter confound_number to n_confound
- Reordered imports
- Fixed documentation
Simplified compcor implementation.

Removed a test for array ordering and two different expressions by
an unique code that performs identically for C or F ordering, though
slightly slower (20%).

Cleanup in test_signals.py
Merge pull request #45 from pgervais/compcor

Added high_variance_confounds() and tests