Accept build requests via api

```
data='{"request": {"repository":{"owner_name":"travis-repos","name":"test-project-1"},"branch":"ansi","config":{"env":["FOO=foo","BAR=bar"]}}}'
curl -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: token $token" \
  -d $data https://api-staging.travis-ci.org/requests
```

Works:

![image](https://cloud.githubusercontent.com/assets/2208/4709222/e04bfbe2-589d-11e4-942e-95ccabb0b7d9.png)

Needs https://github.com/travis-ci/travis-core/pull/393