Implement Hafas REST API

[schildbach]

Recent Hafas installations seem to have a new REST API which we could implement. Currently there doesn't seem a reason to do (unless it can provide data which we don't get via the existing APIs), but I'll put this up as an issue anyway.

Providers known for offering this endpoint: nah.sh and VBB.

Documentation: HAFAS_ReST_Interface_v.1.21.pdf
XSDs: http://demo.hafas.de/openapi/vbb-proxy/xsd

[Anbeku]

Probably needed by #221

[derhuerst]

public-transport/hafas-client#134 adds rest.exe support to hafas-client, so maybe you can draw inspiration. The PR already works with a DB endpoint.