Add GPL exception to license.

The exception allows applets, workflows and plugins extending ilastik
through controlled interfaces to have non GPL compatible licenses.
Change license statements to mention GPL exception.
Add license dialog.
Display license from buildem when running from a binary install.
Binary distribution license path is different for windows.
Merge changes from 'master'
Update new file headers with GPL exception.
Merge pull request #871 from burcin/license

Add GPL exception to license.