added contrib self-executable shell script (shar)

Running:
    `$ contrib/gitflow-installer.shar`
_or_ 
    `$ sh gitflow-installer.shar`
will extract the archive.  Additional logic could easily be added to automagically run `make install`.

To rebuild the archive, without a PGP signature, simply:
    `$ make clean-shar && make shar`

To rebuild with a PGP signed wrapper:
    `$ make clean-shar && make sign`

To verify the PGP signature:
    `$ gpg --verify contrib/gitflow-isntaller.shar`