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
Updated the project repository link.
Silenced an unused parameter warning from posix::sysv_segment::for_each().
Disabled posix::local_socket#{send,recv}_descriptor() on Mac OS X.

This avoids copious compilation errors due to CMSG_SPACE().
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