CMS-1746 Add type definitions for content_tree in RemoteService.ts

Add strict typing for the parameter and result values of RemoteServiceInterface.content_tree()

* Check the RpcHandler method in Java to find the exact values accepted and returned.
* Use optional types (optParam?: string) where appropriate.
* Avoid use of any as much as possible and use concrete types.
* Look at types of space_list() and space_get() for reference.
Merge pull request #256 from enonic/CMS-1746

CMS-1746 Add type definitions for content_tree in RemoteService.ts