Re-organised all Java source code under the java-api subdirectory, which
also contains a Makefile.in that performs the Java compilation. This
makefile is only invoked if ./configure finds a working Java compiler.
Adds a CLI and RESTful API operation for "keyring remove", with simple
test cases. Added the corresponding Java API operation. Updated the
API documentation.
API change: for consistency with RESTful API design, the GET
/restful/keyring/add operation now returns "201 Created" not "200 OK" if
successful.