Initial, untested, commit for audio.

Audio prompts with a maximum duration field are now allowed. This has not yet been tested.
Added the audio input to the survey upload.

When uploading a survey response with audio data, the audio data will be saved as long as it's a multi-part form where the part has a content type that begins with "audio".
Removed useless whitespace.
Fixed the XML name of the audio prompt max duration.
Tested the audio prompt type.

The audio prompt type should now be valid and uploading audio should work. Full end-to-end testing with the phone will need to be done. Also, there will need to be a new API for reading audio files.

Closes #540
The audio prompt has been tested and added.

The audio prompt can now be created and audio files can be uploaded as part of a prompt response. There still needs to be an API to read audio files, but that will be opened as a new issue.

Closes #540