README: Update description and add a 'Credits' section
buffering: fixes for handling large and too large responses

Only require the missing part of the message to be read instead of requiring
the buffer to be large enough to hold the existing parts plus the full
message.

If the buffer is still too small to hold the full value store a 'failure
value' in the response lookup object when needed and make sure we don't set
key in the failure value if the buffer is too small to hold it.
error codes: handle E2BIG and DELTA_BADVAL responses
tests: initial unit tests for omcache, using Check

The tests don't cover most functionality yet, but this commit brings in the
infra to implement more tests and to convert existing manual tests to an
automatically runnable test suite.