Implement edit_agents method to agent api

[joaogbcravo]

From the request: https://groups.google.com/forum/#!topic/go-cd/9dHzfyd9Cc0

"May be you can add edit resources & environments of agent (its undocumented & a new one might be written sometime soon). Explained https://groups.google.com/forum/#!msg/go-cd/LT3ZUEn_kRE/Fw8euL3Tw3IJ"