add library `ace.test.mocks`
make `HasProxy` a public type so that `MockVirtualRenderer` can extend it

this is necessary so that we can inject it into the `new
Editor(VirtualRenderer, Session)` constructor.  The `EditorProxy` needs
to be able to extract the `jsProxy` from the renderer / session
instances.
MockVirtualRenderer : more progress
MockVirtualRenderer : working implementation
editor_test : add a `mockRenderer` and use it in `testFirstVisibleRow` and `testLastVisibleRow`
Merge pull request #41 from rmsmith/mock_virtual_renderer

Mock virtual renderer