Api docs screenboards

changes:
1) made it clearer that dashboard endpoint actually applies to timeboards only

![screen shot 2014-07-08 at 3 41 44 pm](https://cloud.githubusercontent.com/assets/6087353/3515693/00835936-06d8-11e4-9866-7dca5c70fcfa.png)

2) added some examples of using screenboard endpoint and linked to more detailed docs page

![screen shot 2014-07-08 at 3 43 01 pm](https://cloud.githubusercontent.com/assets/6087353/3515697/18bf6ecc-06d8-11e4-8a77-d25d1e45a715.png)