Rhizome Java API: improved form-data headers

Change manifest Content-Type from rhizome-manifest/text to
rhizome/manifest; format="text+binarysig"

Add "Content-Transfer-Encoding" form-part headers to Java API
sent form parts, although not currently checked by Rhizome
RESTful interface
Rhizome Java API: insert anonymous bundle
Rhizome Java API: insert/update empty bundle