Added ReadOnly segment

  + The segment displays a padlock (in patched mode), or 'RO' (in compatibility mode).

  + Added a condition for showing / if that is the only path. The CWD segment earlier showed nothing, which I think is inconsistent.

![screenshot from 2013-09-15 05 56 46](https://f.cloud.github.com/assets/1577944/1144465/cb2e05d4-1d9d-11e3-8ea4-a444ef129464.png)