Genericise the UI flash messaging

- We've started using the messaging system for a few different purposes.
- This pull request makes a simple API to do that.
- Any module can now flash a message at the top of the screen.
- The close button sets a localstorage value to remember the fact you've seen the message.

Eg, this one saying 'hello' with an acknowledgement id of `hi`. 

```
require('modules/ui/message', function (m) {
  var b = new m('hi');
  b.show('hello')
})
```

![hello](https://f.cloud.github.com/assets/84996/1584030/d6903392-5205-11e3-9552-172e21ebae85.png)

Nb. I've still not solved message prioritisation when more than one thing is gunning for the user's attention.