TST: confounds given as filename for signal.clean()

Added a fake confound file: test_files/spm_confounds.txt, and an
associated test.
confounds can be passed as a list to clean()

The "confounds" keyword argument in signal.clean() can now be a list of
arrays / filenames. Proper handling of 1D arrays has be added (it wasn't
working)
Put CacheMixin memory_level back in the good order
Merge pull request #63 from pgervais/file_handling_clean

Simplified confounds loading
Added missing test file.