Use UI classes for prompt

Adds the `tool-panel` and `panel-bottom` classes to the prompt, to match other Atom panels and enable UI themes to work. Also altered the capitalisation of "abbreviation" in the placeholder to match other placeholders in Atom.

![screen shot 2014-08-11 at 15 59 59](https://cloud.githubusercontent.com/assets/1759837/3877739/659f2c00-2168-11e4-9c65-04fbfff5fdb0.png)

![screen shot 2014-08-11 at 16 02 11](https://cloud.githubusercontent.com/assets/1759837/3877747/88953650-2168-11e4-9d15-552a760d552f.png)