Faster unit tests!

One can set the env variable ZPC_TEST_TIME_FACTOR to something smaller
than 1.0 in order to speedup testing. At some really small values, unit
tests about timeouts are likely to fail!

Example:
$ ZPC_TEST_TIME_FACTOR=0.01 nosetests
Run tests on travis 10 times faster.
Bump pyzmq requirement to >=13.1.0
Do not ignore test errors with pyzqm>=14 on travis.
wip