Add a new Poller implementation that uses epoll rather than select

This should scale to a large number of FDs. I still need to do some benchmarking. 

epoll can be enabled with the --use-epoll flag. Additionally for tests setting OLA_USE_EPOLL=true in the environment will enable it. 


Closes #229 
Okay, and I see we get a nice error if we use an unknown option, which will cover that bit. It still might be nice to have a note of which has been chosen.