make sure the resource type exists before actually uploading it
or trying to perform operations on it, returning a 404 if it
does not exist.
update changelog.
Merge remote-tracking branch 'origin/develop' into feature/check-plugin-type

Conflicts:
	CHANGELOG.md
	server/src/main/java/com/continuuity/loom/http/handler/LoomPluginHandler.java
fix merge problems
Merge pull request #326 from continuuity/feature/check-plugin-type

make sure the resource type exists before actually uploading it