wm_authenticate should be used directly in a WM callback
cleanup client API
added proper test for roundtrip

GeneratedcCounterexamples indicate that hmac_api_lib:validate/4
crashes when non-unicode strings are passed to it.
doc some more canonicalization
add early failure in wm_authenticate if there are missing headers
make validation process more robust against non-existant headers
Merge pull request #2 from metadave/dp_proper

added proper test for roundtrip
update proper test for new API
Merge branch 'jc/webmachine-callback-api'