issue/2122-reader-share

Fix #2122 - when sharing a post from the Reader, the shared text now includes the title and uses the post's short url if it has one.

![device-2014-12-18-132833](https://cloud.githubusercontent.com/assets/3903757/5493567/fbd0c6aa-86b9-11e4-9821-75aea474920a.png)

```
// we don't know where the user will choose to share, so enforce a max title length
// in order to fit a tweet
```

So why 200 as max chars? To let the user do its own edit?