Add error and pending status

refs: http://developer.github.com/v3/repos/statuses/

* pending example

![image](https://f.cloud.github.com/assets/19625/570869/47c2bd64-c764-11e2-9b1a-ab8fd4a085ad.png)

* error example

![image](https://f.cloud.github.com/assets/19625/570870/57c3aeda-c764-11e2-92ce-4e5faad1852d.png)

* the difference from failure example

![image](https://f.cloud.github.com/assets/19625/570874/98d3d9fe-c764-11e2-84b7-b8e1472f95c9.png)

Recently i notice Travis CI is posting this status before test run. `pending` status is really useful especially for a long time test (10min ~ 20min or even more).