Integration test on master, not just apiserver.

Moved code from cmd/apiserver to pkg/master.

test/integration/client_test made to use a master object,
instead of an apiserver.Handle.

Subsequent PRs will move more handler-installation into
pkg/master, with the goal that every http.Handler of a
standalone apiserver process can also be tested
in a "testing"-style go test.

In particular, a subsequent PR will test
authorization.
Nit: I think you can do this all in one line like ```s := httptest.NewServer(mux)```