Add a Timeout

[naysawn]

I'm finding that sometimes the Plivo API isn't responsive. In this case, it can cause the app to hang waiting for a response.

I think the simple fix would be to add a timeout parameter to the RestClient::Resource.new instantiation.

http://rubydoc.info/gems/rest-client/1.6.1/RestClient/Resource#timeout-instance_method

[tsudot]

Could you please send a pull request? The last PR could not be merged because master has diverged.