Link Twitter handles and hashtags in Twitter onebox

Link all the things!

Besides linking twitter handles and hashtags, this PR also adds tests for `TwitterOnebox#parse`

![link-handles](https://f.cloud.github.com/assets/65323/652458/f516914c-d4a8-11e2-8e37-5b96ec4c91b2.jpg)

![link-hashtags](https://f.cloud.github.com/assets/65323/652459/f7f6de26-d4a8-11e2-9f7c-a3e8c40072e7.jpg)