4.1

This is the pull request for v4.1.

- [x] #153 Definition dumper (needed for #143)
- [x] #143 Better exception messages
- [x] #138 Container-interop compliance
- [x] #157 HHVM support
- [x] Better Symfony 2 documentation
- [x] Documentation update
- [x] Change log update
- [x] News entry