RPM build reworking

The rpm build system has be rewritten to create signed RPMs in a yum repository structure. It's been written to allow automated builds via Jenkins and now:

- Produces packages for CentOS/RHEL/SL 6.x & Fedora 19 x86_64 & i686 . Other architecture may work but are untested due to lack of suitable hardware. 
- Results are in the structure ./ghost_release_number/distro/arch/*.rpm , e.g. 0.3.3/el6/x86_64/ghost-0.3.3-3.el6.x86_64.rpm . This allows a symlink to be moved around to point to the latest version on the real web server
- Resulting RPMs can be signed via a GPG key. The GPG passphrase is passed via an env var 

There is more discussion in #8 

I'd appreciate some testing of what's there to make sure it's working for others. It's ok for me, but they I wrote it :)