Implement release tagging and version checking for client side

When you prepare to deploy a new version, do this:

bin/hypothesis tag

This will update the h/js/release_tag.coffee file with a timestamp/name
for the current release. (You should commit this file to your branch, too.)

When the application is running, both halves of the code
(the one injected into the host document, and the one running in the sidebar)
see this file, and read out the releast tag.

In cases when two frames somehow end up running different versions of the code,
this difference is now detected.

This fixes #702.

Note the current implementation does not check the server version, only the
version of the two halves of the client code.

About the server version, issues #278 and #513 are still pending.
When those are implemented, the same mechanism can easily be extended
for checking the server side,too. (The stubs for it are already there.)