Make do_request's params payload generic

do_json_request should handle encoding params to json, not do_request

this means we can then send multipart post requests with do_request
Update places_kml to expect a response object from get_raw 

We actually have a test for places_kml now too \o/
Add post_multipart method to JsonClient

This changes the existing behaviour of do_raw_request
to return the entire http response, and not just the
body. 

This makes get_raw more consistent with get_*/post_*
methods in JsonClient in returning a response. 
(However, get_raw does not return a GdsApi::Response 
due to the assumptions it makes around parsing JSON.)

From a search of the code, the only method using this
is the Imminence API's places_kml method, which I've
updated in the preceding commit.
Add post_multipart to the delegated methods list
Add asset manager api adapter
Allow attributes other than the file to be sent to the asset manager
Add method to get an asset from the asset manager

Includes a test helper for stubbing the asset manager in tests.
Add rdoc comments for the asset manager adapter
Merge remote branch 'origin/asset-manager'