Structured exclusion rules internally, and on the popup and options pages

Follow up to #1143 (and the aborted #1150).

This implements structured exclusion rules internally, and on the popup and options pages.

Comments and issues:

- There's no change in functionality here (over #1143).  Just changes to the internal representation and the popup and options pages.

- Check out the browser-action popup.  It's pre-populated and updates its state depending upon what's currently in the input fields.  It's not quite possible... but I did consider whether the popup is now sufficiently functional that we could remove exclusions from the options page completely.

- On the options page, exclusion rules are now structured elements (as opposed to flat text).  Since order matters, it would be nice to have `Move Up` and `Move Down` buttons.  However, to my eye, it's pretty cramped already, so I left this out.  There is no `Add Rule` button.  A new (empty) row appears when you hit `Save Options`.

- I originally implemented caching for `Exclusions.getRule()`.  But it turns out that the hit rate is low (about 20%, for me).  Which means that the cache would become too large too quickly, and be of little benefit anyway.  So I removed that and implemented a `RegExp` cache instead.  We still do a linear scan on each change of tab, but it is no longer necessary to parse JSON and build RegExps each time (as was previously the case).  

- I didn't want to put lots of exclusion-rule logic in `options.coffee`.  So I added a new type of option, a *self-handling option*.  There are various ways in which this could be done, so perhaps somebody should take a closer look at it.

- The PR includes code for migrating from the old `excludedUrls` setting to the new `exclusionRules` setting.

- The PR plays nicely with the syncing of settings across multiple chrome instances.

- There's an awful lot of code: sorry about that.

- Comments from @philc in #1143 and @deiga in #1150 have been taken into account: Thanks!

As ever, comments are welcome.

Please try it out.  There's quite come change to the user interface for configuring exclusion rules.

Remarkably, `keyMappings` seems never to have had a default value!