Exchange simulator: switch from epoll to libevent

In order to support market simulator on various
platforms a cross-platform techniques should be
used. In particular, epoll is not present on such
a platform as Darwin but libevent fills the gap.

Signed-off-by: Marat Stanichenko <mstanichenko@gmail.com>
Merge pull request #42 from mstanichenko/libevent

Exchange simulator: switch from epoll to libevent