Pass event object into event handler in Guest

This fixes an issue where an exception is raised when the onAdderMouseup
handler tries to call preventDefault on a non-existant event object.
Add tests for onAdderMouseup handler
Merge pull request #1658 from hypothesis/fix-adder-event-handler

Fix adder event handler in Annotator.Guest
WIP: working (though hand-built) add-on attempt
Add required `browser` param to `extension` command

So we can support more than just Chrome.

Only `chrome` works (as of this commit), but
`firefox` support is coming next.
Add Firefox extension build option

Mostly a copy / paste job (for now) from the
Chrome extension.
Keep Firefox app.html loading from site vs. addon

Firefox throws event errors when trying to open the
sidebar iframe with `app.html` hosted at a `resource://`
URL. Bringing it in from the remote works fine, however.
Switch to sleeping icon for FF add-on button
Removed resources the builder handles
Remove unused favicon1.ico reference
Use const instead of var
Fix extension build intructions in README

Also update in code docs.
Remove manifest call; copy bootstrap & destroy
Fix Firefox Add-on ID

Srsly!1!...who wrote this readme section... >_<
Embed Web Fonts into the icomoon.css

Firefox does not like loading fonts out of resource:// URLs.
This gets around that.
Move icomoon.css into styles; Remove unused fonts
Remove duplicate copyfile for Chrome

Also reorder bootstrap & destroy copyfile in Firefox.
One build [script] to rule them [both]

...and in the darkness bind them.
PEP8
Catch the ready state for the tabs collection

Check the current tab.

Covers 90% of the state scenarios.
Still some slipping through though...