support rest.exe endpoints

[derhuerst]

rest.exe HAFAS endpoints have their own URL and require authentication via a token uniquely issued to each customer/user. While they follow the semantics of the mgate.exe endpoints, their responses have entirely different field names and sometimes different nesting levels.

Currently, the mgate.exe field names are hard-coded into the formatting/parsing code of hafas-client (it allows limited customization by overriding formatting/parse functions). We have several ways to support the rest.exe protocol:

nr	approach	complexity per client	duplicated semantics-related code	coordinating overarching work
1	write an entirely new client without sharing code	little	a lot	harder to coordinate
2	abstract formatting & parsing layer, used by both clients in one repo	more, becuase intermediate format	almost none	as before
3	add rest.exe-specific formatting & parsing functions, some of them re-using the mgate.exe	a bit more	a bit	as before

With all three options, one should be able to conveniently specify in an endpoint profile what protocol the endpoint speaks. Also, I should be able to override specific formatting/parsing functions, just like I currently am with mgate.exe.

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I prefer option 2, but it requires significant refactoring work, and I'd also like to merge #132 first, so that we don't accidentally break the mgate.exe code. @juliuste What do you think?

I went ahead with option 3 in this PR, because it was the most straightforward way to write it step by step. This doesn't mean that we have to keep it this way though! Because we try to avoid breaking changes in hafas-client, it seems appropriate to fully cover at least one rest.exe endpoint before we merge/publish the code.

still to do:

• cover all features of the API
• squash/clean the commits
• unify parse and parse-rest? (see above)
• find a way to write a profile for rest.exe
• some bugfixes
• unit tests, end-to-end tests (see also add unit tests #132)
• documentation
• adapt the "writing a profile" guide
• rename $TOKEN to $HAFAS_TOKEN? (breaking change)

[juliuste]

(Will rebase to current master and publish a new tag 5.1.1-rest.alpha)

[marzieh-kazemi]

id is missing on the trip returned by .trip(id).