Add availability endpoint to docs

![image](https://cloud.githubusercontent.com/assets/6557731/4058397/60edfe6a-2dd5-11e4-9a16-2ef48034e9e8.png)

This is slightly more complex actually, as each number represent a status as:
```
available: '0',
booked: '1',
tentative: '2',
unavailable: '3'
```