Properly indent StaticRequestHandler
Improve handling of non-existing static files

Instead of SEGFAULT, we rather just send a nice error messaage.

Further, this abstracts out a few more parts into AbstractConnection,
so that a dynamic_cast isn't necessary any longer.
Merge pull request #107 from bastih/feature/improve_static_handler

Improve static handler to not segfault on non-existing files + some general improvements