Clean up histogram

Also, a temporary fix to the subarray range issue is hiding in here
Experimenting with the contrast GUI
Merge branch 'master' into rsr/contrast
Merge branch 'master' into rsr/contrast
Plot images in existing canvases (fix #3)
Add new test functionality
Satisfied with layout
Small cleanups to tile example
Add convenience functions for window management
Set slider value properly (for int sliders)
Entries set sliders and rescales the contrast on the image

Now for the sliders to trigger an update of the image...
Sliders now control image contrast (live)
Cleanup

Make the bindings similar
Merge branch 'rsr/contrast'
Contrast GUI: fix inexact error for integer images
Work around SubArray limitations by taking the data
Small improvements to initialization, display, and callbacks of contrast GUI
Fix for sliders now returning floating point values (JuliaLang/Tk.jl@e5bb68fbba)
Don't let the contrast values cross
Add imread() fallback that uses file open dialog