Issue 79 receive timeout should be configurable

@carpnick I tweaked your PR a little bit WRT to keeping the http client receive timeout close to the operations timeout. LMK if you foresee any issues with this.

- The http receive timeout still defaults to 3600s
- When setting the operations timeout the http receive timeout will be operations timeout + 10s
- You can optionally override the http receive timeout when setting the timeout
Theory here being 10 seconds is long enough to catch the 99% use case where it takes less than 10 seconds to send back a WINRM operation timeout error over the connection?

If so I am good.