extracted Errors
skip relative schema imports for now

if someone finds a specification for dealing with those,
please let me know.
BindingOperation#input now returns header and body
updated the changelog [ci skip]
removed the dependency on HTTPI /cc #27

Since Wasabi only does GET requests, it only needs a very simple
interface for sending HTTP requests which works great with testing
and allows you to use any HTTP client via a very simple adapter.

see spec/support/http_mock.rb for an example.