[RFR] Add project configuration to API model

- [x] Add configuration item
- [x] Add previous analyses hashes
Method removed. Retrieving configuration is no more relevant, as we get the `call` method now.

 Yet, as I implemented a [Guzzle backoff plug-in](http://guzzle3.readthedocs.org/plugins/backoff-plugin.html), I now pass the `Client` to the `getApi()` method (see [this file](https://github.com/sensiolabs/insight/pull/25/files#diff-8228b02b8072a7d7d6c2e6e3eafe352aR43)).