Feature #3347 REST-ified FileBucket

FileBucket Files have been reimplemented as an indirector terminus so that
they can be transmitted over REST.
The old Network::Client.dipper has been replaced with a compatibility later
in FileBucket::Dipper that uses the indirector to access filebucket termini.

Slightly revised patch:

• No longer allows nil contents in FileBucket outside of initialization
• Uses File.exist? instead of the deprecated File.exists?
• Tweaks JSON serialization and de-serialization to include "path"

Deferred issues:

• Feature #3371 "FileBucket should not keep files in memory".
• Feature #3372 "Replace FileBucket Dipper with more idiomatic calls"