Increased tolerance level in a test assertion.

The previous tolerance level caused the test to fail on some machines.
Fixed a bug in signals.clean()

The "detrend" boolean argument in clean() is an easy way to add a trend
as another confound to remove. It must be exactly equivalent to adding
a trend in the "confounds" array. This was not the case before this
commit.
Merge pull request #49 from pgervais/clean_confounds_fix

Fixed a bug in signals.clean()