Recover gracefully if the pid/socket files are obsolete

E.g. if the server process has received SIGKILL and so didn't get to
cleanup.

Fixes #7
Don't alternate the purpose of server.accept

This originated as an attempt to fixed #2, #4, but it didn't help.
However, I think it's a good idea anyway in order to prevent the
alternation potentially getting out of sync.
The IO class can be specified directly in the recv_io call
unit tests no longer depend on mtime resoultion
run unit-tests and acceptance-tests separate to get faster feedback
Merge pull request #10 from senny/tests_do_not_depend_on_mtime_resolutions

unit tests no longer depend on mtime resoultion
Merge pull request #11 from senny/run_test_suite_independent

run unit-tests and acceptance-tests separate to get faster feedback