Implement client settings, which the server is fetching upon initialization

[mfelsche]

Define a settings format which the server expects the client to deliver. The main setting we should expect is the PONYPATH which must include the path to the stdlib source location. Currently we ship the stdlib of the supported ponyc version. This helps keeping everything consistent, and in sync, so we have no different stdlib while working with a libponyc from another, possible inconsistent version.

But customizing the $PONYPATH with client settiongs might be helpful nonetheless, as a start.