Activation commands

This PR properly integrates the new global command registry (`atom.commands`) with package activation events. It does the following:

* Switches the syntax of `atom.activationEvents` to `atom.activationCommands`. This hash has mandatory selectors mapping to strings or arrays of strings as follows:
```coffee
  activationCommands: {
    ".workspace": [
      "project-find:show",
      "project-find:toggle",
      "project-find:show-in-current-directory",
      # ...
    ]
  }
```
* Uses `atom.commands` instead of `$::command` to listen for package activation. This means we won't need to use a deprecated API to support this behavior when we deprecate `atom.workspaceView` and `$::command`.
* Ensures that commands registered with `atom.commands` are invoked only once even when the activation command is replayed after activating the package.

This is delicate work but I'm pretty sure it preserves existing behavior. Specs pass and I tried a handful of baked-in packages without issues. One nice thing: Because the command registry uses the [capture phase](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget.addEventListener) of events, we don't need to disable the jQuery event listeners anymore to prevent them from being invoked twice. We stop propagation of the activation command and reinvoke it after activating the package. It's this second invocation that ends up calling listeners registered via jQuery.