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
Added a test for concat_niimgs
Optimized memory usage of concat_niimgs

Dropped usage of np.concatenate(), reducing memory usage by a factor
of 3. A test for inconsistent shapes has been added, with a clear user
error message.

Execution time of concat_niimgs() is similar, possibly slightly
shorter than for the previous version.
Merge pull request #46 from pgervais/concat_optimization

Reduced memory consumption for concat_niimgs
Merge pull request #45 from pgervais/compcor

Added high_variance_confounds() and tests
Merge branch 'master' into roi