WIP : added poll_fd and test for the same on branch kf/fdwatcher

  added ```test/pollfd.jl```. Using a unix ```pipe()``` to simulate watchable fds. Consequently the test only works under unix currently. Tested only on Ubuntu 12.10

Issues noticed:

- sometimes the timer is going off earlier than expected, hence the code in ```stream.jl``` loops around the yield. This has to be removed once the issue is fixed.

- ```sleep()``` in ```test/pollfd.jl``` is exiting earlier than expected.