Defined the rfc791::ipv4_address class.
Imported the Catch 1.0.27 unit-test framework's single-header build.

Source: https://raw.github.com/philsquared/Catch/master/single_include/catch.hpp
Migrated from Boost.Test to the Catch unit-test framework.

This removes Boost as a build dependency, eliminates weird problems
with `make check` on certain platforms, and generally makes life more
pleasant.

See: https://github.com/philsquared/Catch