Deduplicate m.route and m.redraw logic (#2453)

- Remove appropriate route change subcriptions when a root is removed
  via `m.mount(root, null)`.
- Don't pollute `onpopstate` and friends - use standard event listeners
  instead.
- Simplify and streamline subscriptions, in preparation of adding a
  `remove` parameter to `m.mount`.
- Change the redraw internals to redraw immediately, with ability to
  cancel via returning a sentinel.
- Change `"bleeding-edge"` for `m.version` in `next` to instead just be
  the latest `m.version`. (If you're using `next`, you should know what
  you're in for.)
- Update tests to be aware of these changes. (Some were failing for
  subtle reasons.)
- Drive-by: remove some uses of `string.charAt(n)` and use `string[n]`
  instead.

index.js

@@ -21,7 +21,7 @@ m.parseQueryString = require("./querystring/parse")
 m.buildQueryString = require("./querystring/build")
 m.parsePathname = require("./pathname/parse")
 m.buildPathname = require("./pathname/build")
-m.version = "bleeding-edge"
+m.version = require("./package.json").version
 m.vnode = require("./render/vnode")
 m.PromisePolyfill = require("./promise/polyfill")