Support running with zero-configuration

[fredemmott]

We expect this project to be replaced by a future zero-configuration built-in autoloader; to prepare for that, we should work on making this work with zero configuration to uncover any issues that will need to be addressed by the future built-in one.

[lexidor]

The minimum configuration required is:

{ "roots": [], "useFactsIfAvailable": true }

¿I think ext_watchman was the future zero-configuration autoloader you were talking about?

[fredemmott]

There's also .hhvmconfig.hdf for now, but that should be made optional too