replaced rendering of contributors from js to real sub-request

Another PR with does some cosmetics.
I have seen that the contributions area is filled from a mix of js and an additional Symfony controller. 
The GET request to GitHub's API is done on every PuPHPet request and I wanted to cache it. Therefore I have implemented a CacheWarmer which fetches the contents from the API and caches it in one cache file `%kernel.cache_dir/contributors.json%`. An every `cache:clear` the list is updated now.
Within the template the contributors area is filled via a real sub-request. A new Repository class tries to get the contents from the cache first. If nothing could be found it does a fallback on the real API request service.

*Attention:*
I had to install `php5-curl` first on my fresh box as I use curl for fetching the data from GitHub. Please go sure that curl is also installed on your machines.