ImageNotLoadedException now stores the item that was requested.  This
gets passed to the image loading dialog manager without having to know
if it was a reading or a source.
Renamed get_current_displayable_item to get_current_displayable_image. get_current_displayable_item now returns the source/reading associated
with the displayable image.
on_image_loaded now works with displayable items which can be readings
or sources.  Triplet grid finished downloading callback now generates an 
image loaded event.
Enabled triplets in display menu.  Very basic triplet grid is now renderable.
Moved image viewer manager to its own module.
Merge branch 'master' into triplet_controllers
Moved workload module into models package.  Split the 
StatefulCollection class out into its own collections module.
Merge branch 'master' into triplet_controllers
Fix launching of debug mode; need to create inspection tool before 
starting the wx app main loop.
Image view manager is now part of the ApplicationView instead of 
MainFrame.
ImageViewManager now creates the matplotlib figure canvas and its is
shared by the singlet and triplet viewers.
Added method for submitting triplet download requests on retries.
Image loading dialog is displayed when switching between singlet and 
triplet mode if the image isn't already loaded.
Merge pull request #198 from drusk/triplet_controllers

Switching to and from triplet view