CMS-1835 Add type definitions for account_get in RemoteService.ts

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

* 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.
Merge pull request #280 from enonic/CMS-1835

CMS-1835 Add type definitions for account_get in RemoteService.ts