use poll() instead of select()

use poll() instead of select()

poll() is much nicer and more efficient API than select(), additionally
it doesn't require bitsets of 4096 bits which need to be initialized before
each call, improving CPU load and, best of all: ain't prone to stack
overflow should the user decide to change his OS's fd limit above 4096 and
having a fd above that number in use.

fixes #13
closes #16