[PIE-4] plugins should call reply.continue(), not reply()

if there is nothing to do, reply.coninue should be called. Calling reply
with no arguments starts a new reply and replaces the old one. Which
is an empty response
[PIE-4] Alllow api plugin to be registed multiple times
[PIE-4] Support for field level hydration.

attribute mapping through data and object data
[PIE-4] no content response doesn't need to send any data
[PIE-4] API level serialization and resource hydration.

remove serialization / desieralization from resource method by default.
the API plugin handles it. But the generic serialization methods are there, and can be used.

Also adding support for resource hydration to convert incoming data
into usable data structures.