Add active_encode support to hydra-derivatives

[cjcolvar]

The ActiveEncode runner allows using ActiveEncode within hydra-derivatives to generate derivatives using the adapters supported by ActiveEncode. Currently Amazon Elastic Transcoder is the only production ready adapter available but prototypes exist for zencoder and shingoncoder (FFmpeg through ActiveJob). The key differences between ActiveEncode and hydra-derivatives are:

• ActiveEncode can produce multiple derivatives of potentially different formats (video derivatives, thumbnail images) from a single invocation
• ActiveEncode does not necessarily operate on local files
• ActiveEncode returns a url to the derivatives it creates and not a bitstream
• ActiveEncode allows for polling to get updates on its progress

In support of this a source file service and output file service have been added. The RemoteServiceFile takes the :source option and calls a method of that name on the source object to determine the source file path or url to send to the processor. The PersistExternalFileOutputService creates an ActiveFedora::File with a url to the derivative output instead of storing the actual bit contents of the derivative in Fedora.

Documentation is added for configuring and using the ActiveEncode runner with Amazon Elastic Transcoder which seems like the main use case for users at this point.

Thanks to @val99erie for a lot of this work.

[cjcolvar]

@cam156 Thanks for your comments. I think I responded or handled them in the last commit I pushed. @mjgiarlo Do you mind reviewing as well since you looked over things while they were WIP?

[mjgiarlo]

@cjcolvar You bet.

[cjcolvar]

@mjgiarlo I think I took care of everything that isn't documentation or comments changes. I'll come back to those later today/tomorrow.

[mjgiarlo]

@cjcolvar OK, thanks! Ping me when it's :green_heart: and all ready to go.

[coveralls]

Changes Unknown when pulling ee0ad3fcce5729ed4b185beed13b4a8fe2380522 on active_encode_dev_con into on master.

[coveralls]

Changes Unknown when pulling ee0ad3fcce5729ed4b185beed13b4a8fe2380522 on active_encode_dev_con into on master.

[coveralls]

Changes Unknown when pulling ee0ad3fcce5729ed4b185beed13b4a8fe2380522 on active_encode_dev_con into on master.

[cjcolvar]

@mjgiarlo Ready. Any additional changes you'd like to see?

[mjgiarlo]

@cjcolvar Taking one last look. Thanks for all the changes, and yr patience!

[mjgiarlo]

:tada: Thanks for the contribution! Great work. (Do you want/need a new release now that this is merged?)