check zmq version at top-level kernel import

maybe someday this won't be necessary, but it is now.
move DEFAULT_STATIC_FILES_PATH to frontend.html.notebook

and move the friendly version checks to notebookapp.py

DEFAULT_STATIC_FILES_PATH is now accessible without pyzmq/tornado/jinja being importable.
If someone tries to use old pyzmq or tornado directly with handlers,
the version check won't happen, but that's probably the right thing to do anyway.
remove workarounds for no-longer-supported pyzmq versions
move check_for_zmq to utils.zmqrelated
Merge pull request #3105 from minrk/static_path

move DEFAULT_STATIC_FILES_PATH to frontend.html.notebook