FSL ebuild 

The FMRIB Software Library (FSL) is a neuroimaging data analysis suite written and maintained at the University of Oxford. It is widely acknowledged as the second most popular neuroimaging data analysis package (after SPM). In contrast to SPM it is properly versioned by upstream, does not depend on non-FOSS stuff - and offers some functions which SPM does not. 

FSL also integrates/is used by nipype

As part of my current neurogentoo-effort I believe FSL would be an awesome addition to our overlay and Porateg in general.

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Now, some issues:
* The ebuild puts everythig under ```/usr/lib/fsl/``` - this was dome by https://github.com/christian-burger to whom I am very grateful for helping with the ebuild. Still, I am not sure this is the best place for the files.

* The launcher is, thus, under ```/usr/lib/fsl/bin/fsl``` - should we at least move that to ```/usr/bin``` ?