API Updates

Hi, 

We needed some extra functionality from the API which we added to this fork. We think that these changes can benefit other developers.

Features Added: 
1. Retrieving the list of available urls for a client: Client.GetAccountInfo
2. Functionality around CRUD operations of custom contact fields.
3. Fixed README file. It had {} around the /id. It confused us, because we thought it was a token that we needed to provide.