Add Image_question to the activty_runtime_api
blob.rb: Add mimetype,  file_extension, checksum

Learner.rb: Add assocation to lightweight_blobs <=> blobs
Also: add methods to pull content from URL, generate checksum
&related tests.
ProcessExternalActivityDataJob -- adding image question answers.

proceess_external_activity_data_job.rb: support  image_question
&update spec / features