Merge pull request #346 from reaperhulk/mark-tests-2

Add Actual Test Marks
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