Do not check for translations to already-translated items, unless explicitly requested.

[davidmason]

Normal polling for translations should skip items that are already fully translated, to reduce unnecessary network traffic.

An alternative polling operation should be available to check all items, including fully translated items, for use in cases where a bug is found in an accepted translation and it must be updated.

Use of etags may be possible here.

[davidmason]

Added etags to prevent download of documents that have not had any translation changes since last translation download.

See: f68ee55