Allow installing/running from relative directories, fix views & error handling

This PR introduces a few improvements around how views and assets are handled, to allow running git-pretty-stats from within a relative path; this is achieved by prefixing asset references with `app.request.baseUrl`, which contains the base url for the current request.

In the process I also cleaned up views to make use of Twig's block functionality, which is cleaner over the original `include` method, particularly when structuring the layout, and moved error handling to Silex's `Silex\Application::error` method, which should provide more reliable behavior.

Cheers!