Set timeout at the request level

[bgroupe]

Hi and thanks for making Frisby :)

I know there is an open issue for documentation but it took me some time and frustration to figure out that you cannot specify timeout as an option to .fetch because it is read from the FrisbySpec. It would have been nice to have this called out in the documentation. It would also be nice to be able to set timeouts with .fetch for individual requests.