README.rst: updates for requirements and compatibility

* add platform requirements section
* add more notes about ketama
* and coverity badge
tests: more verbose logging and ability to select case in args

Set Check's output verbosity to VERBOSE up from NORMAL (prints a summary of
all test cases at the end of the run) and make it log test execution on
stdout.  Also make sure stderr and stdout are linebuffered to make sure the
output is at least somewhat ordered.

Also made test_omcache interpret the first argument (if any) as the name of
the test case to run.  The default is to run all cases.
test_suspended_memcache: sleep 0.1s between retries after re-enabling server

Sometimes it'll take a while for the suspended memcached to start talking to
us again, sleep a bit between retries to allow it to recover.  Should fix
transiently failing test_suspended_memcache.