Add packaging/, that allows for a folly SRPM to be created

The SRPM can be built, as long as the double-converion-devel RPM is
installed. The change include a spec file for the SRPM, and a bash
script that can run on a Fedora-system to generate the srpm in
a user-provided directory.

For example:

    ./packaging/build-srpm -o .

Will generate an SRPM in the local directory, for example:

    folly-2014.01.17.092909.gitfc78584-1.fc19.src.rpm
Depend on a system-provided google-gtest library