Added support for request timeout

[kwaazaar]

Currently the webrequest object (.net object used to make the actual api calls) does not have a request timeout explicitly set, thus using the default timeout of 60 seconds. However, for many calls this is too much, especially when a service using the client itself uses a smaller timeout. This PR makes it possible to configure this request timeout on a Client object.

[samwierema]

@kwaazaar thanks for opening a pull request (and sorry it's taken so long for us to reply). We'll review your code next week and get back to you with any feedback we have.