Implement new RequestHandler API
Merge pull request #512 from lucasr/request-handler

Support custom bitmap loading with a new RequestHandler API
Prioritize injected RequestHandlers over defaults
Merge pull request #654 from timehop/fix-requesthandler-override

Allow injected RequestHandlers to override the defaults
Propogate load source when using InputStream.
Merge pull request #661 from square/jw/correctly-pass-source

Propogate load source when using InputStream.