3551: Fix source tree layout.
* Move $GOPATH out of the source tree. ($GOPATH is a workspace. Go
knows how to import source trees into $GOPATH/src/.) This avoids
replicating the Go workspace structure inside our source tree, and
allows Go programs to import our libraries the normal way.
* Add Go SDK documentation page showing how to import.
* Rename keep to keepstore.
* In Python tests, we assume keepproxy and keepstore binaries are
already installed in $GOPATH/bin/ rather than guessing how you want
them to be built.
* Delete go.sh wrappers. Dependencies are already installed
automatically by Go. GOPATH is left alone.