Module requires curl library but does not check for it

I just installed Hotspot and Ewt on a box that didn't have the curl extension installed. I didn't get a warning until I browsed to a page that called ewt_client() and the result was a WSOD. Oops.

Attached patch adds an installation requirement and wraps the EWTClient class in a check for the curl library, so users get a friendlier error.