Upgraded yaml-cpp API from v0.3 -> v0.5.

The new API is far more flexible and automagic than the old one which
will be useful for future development. I changed the main SYN++ routine
as little as possible (just enough to work with the new API), so there
is probably room for improvement there.

Both v0.3 and v0.5 adhere to the same YAML standard (1.2) so users
should notice no difference at all.

Most of the new code here is from the handful of Boost headers that the
new API requires. (The Boost tool 'bcp' makes resolving header
dependencies trivial.) Despite the huge amount of new code, compile
times are hardly any longer than before.