Removed use of CZMQ_EXPORT in API

Fixes #71
Finished thin client stack

Testable with external broker. I've made clients/zpipes_test_client
as a test case, which runs successfully. Note that:

- broker has to run on real IPC endpoint (like /tmp/zpipes-local)
  for now.

- broker must be running before client starts, as libzmtp does not
  yet do lazy connects.

Fixes #49 again.
Server timeout is now in msec

There is one test case where I set this to 200 to test the timeout
mechanism. It was wrongly defined in seconds in zbroker.cfg. All
seems to work now.
Merge pull request #72 from hintjens/master

Removed use of CZMQ_EXPORT in API