added command line setting to specify the path to the go binary. useful ...

...if you write appengine tests, here you must use "goapp test" instead of "go test". When starting "goconvey" there is a setting "-gobin" to specifiy the path. in my local installation i have the current version of the appengine SDK installed in `/home/usc/tools/appengine/go_appengine_current` and now i can run goconvey to use "goapp":

goconvey -gobin /home/usc/tools/appengine/go_appengine_current/goapp