Add a Bootstrap password form for protected posts

### Before:

![image](https://cloud.githubusercontent.com/assets/1671933/3677589/e8876d92-1292-11e4-8dbf-49ff534ffca2.png)


### After:

![image](https://cloud.githubusercontent.com/assets/1671933/3677603/04c9d1e8-1293-11e4-9c19-388656ba253f.png)