Replace FFT filter by Butterworth filter

And replace default value for low and high frequency un signal.clean
by None instead of False.
Put warning into butterworth filter if values are wrong.
ENH: Minor fixed. Butterworth takes nD data.
ENH: Haxby downloads needed files only
BUG: MD5 sum loading in Haxby
ENH: now uncompress well .gz files in datasets
BUG: Error case wrongly handled
Add back bz2 filetype
Merge pull request #38 from AlexandreAbraham/butterworth_filter

Replace FFT filter by Butterworth filter
Small typo in plot_simulated_data.py
BUG: compute_epi_mask recomputed if n_jobs changed
BUG: Fix last commit bug
.tar.gz was not extracted
Fix dataset fetch by closing a temporary file prior to moving.
Merge pull request #40 from jmargeta/dataset_fetch_fix

Fix dataset fetch by closing a temporary file prior to moving.
Added icbm152_2009 dataset.
Corrected a typo in plot_rest_clustering.py
Small fixes in tutorial page (documentation)
Documentation fixes

Mainly langages corrections and typos.
Added hyperlinks to articles on bibliographic references.
Fixes in test

- Workaround for a Python 2.6/2.7 bug, that was triggered by check_niimgs()
- Make tests/test_signals.py work with numpy 1.3.0.