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.