Tests for parser

Built on top of #28.

PLEASE, DO NOT MERGE UNTIL #28 IS MERGED.

- https://trello.com/c/ObThuTMy/1755-proper-testsuite-for-api-blueprint-api-app
- https://trello.com/c/CuPDy6Ve/1739-api-blueprint-api-error-handling-proper-http-codes
this is a subject of further development - for now, we support only one version, but in the future it should work like this:

- if client requests a version explicitely, it should get that version in response (if we don't support it anymore, client will get the closest match I guess...)
- if client doesn't say anything about version, we can either refuse the request, or return the oldest and firstmost supported version - for now we somewhat decided with @Almad the client won't be refused, so the latter applies