Must* and HTTP functions

Sorry for having two features in one PR. The second feature requires the first one.

I have added a wrapper for simple HTTP GET's: HTTPGetString(url string) returns the HTTP GET response body as string and optionally an error.
That function is wrapped by MustHTTPGetString(url string), which doesnt return a second value (error), but instead panics when an error occurs. The panic is recovered at *Project.run(..) so it is nicely printed without stacktrace.

I think the Must* approach can be used for multiple other godo helper functions.