Migrate most tests to robolectric
Add runner implementations and tests
convert some robolectric tests to junit tests
Add an engine with dependencies.
Preliminary working Engine
Check disk cache on single background thread.
Make SourceResourceRunner prioritized
remove redundant test
Move runners entirely into Engine and add Listener
Add resource reference counting
Support simultaneous image and video requests
Delete corrupt/invalid images from disk cache.
Add support for transformations.
Functioning flickr demo
Remove ImageManager
Use class for keys rather than StringBuilder
Fix bug in multi type request coordinator
Recycle resources if cancelled on finish.
Cache when we're unable to create model loaders
Replace requestContext with LoadProvider.