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.
added some docs
added time slice handling to OpGraphCut
Display-Thresholding added in LabelingGUI, depends on volumina changes
Display-Thresholding added in LabelingGUI, depends on volumina changes
Display-Thresholding added in LabelingGUI, depends on volumina changes
moved callback connections to end of __init__
Merge pull request #882 from burgerdev/issue-878

[ThresholdTwoLevelsGui] moved callback connections to end of __init__
PixelClassification: show/hide the thresholding tool button depending on whether or not the data has multiple channels.
PixelClassification: Thresholding button is shown or hidden based on the layer type, not the number of channels.
Tooltip for Window Leveling button
CarvingDrawer: Added thresholding toolbutton
based on the grayscalelayer-flag window leveling it is determined if the window_leveling feature is active
Merge branch 'master' into thresholding-documentation
adjusted graph-cut unit tests to canonical axes order
adjusted OpGraphCut to canonical axes order