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
This is 60 minutes I believe as coded: https://github.com/nahi/httpclient/blob/master/lib/httpclient.rb#L339

Minor comment update.