Add packaging/, that allows for a double-conversion SRPM to be created

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:

    double-conversion-1.1.5.date2014.01.17.164630.gite33f459-1.fc19.src.rpm