Include teh name of the backend in the error message
Update test
Move stuff around and coverage
Merge pull request #347 from alex/better-error-emssage

Better error emssage
initial commoncrypto backend. hash only
some tests & temporary skip checks
remove some unneeded internal detail for commondigest structs
skip commoncrypto if not on darwin
hashcontexts have algorithm as a public property
initial backend docs
switch to namedtuple, add assert checks
remove unneeded skips now that markers are in place
try to load a list of backends. WIP
remove some unneeded methods now that we have markers
add commoncrypto marker