AFHTTPRequestOperationManager: Added timeoutInterval Option to all HTTP Verbs

It's a hassle to need to subclass, or create categories for `AFHTTPRequestOperationManager` simply to set a timeout value on the underlying HTTP request.

This pull request:

* Adds the timeoutInterval parameter to each HTTP verb.
* Uses DRY code, calling an HTTP verb without a timeout uses the default 60.0f timeout.
* Creates tests for AFHTTPRequestOperationManager.