Added a test for poll_fd

- 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.