Remove unused Makefile

The install.sh script appears to be the documented install script, the Makefile
depends on git-flow which is invalid here and this only serves to confuse new
users attempting to install.
Pass args on to gitflow-installer.sh

Allow install, uninstall args to be passed through
Don't require root if install dir is writable

For many users (particularly OS X users if they've setup HomeBrew the
recommended way) the default install prefix of /usr/local/bin will already be
writable and for users specifying an alternate prefix like $HOME/bin it will be
as well.  So don't require sudo usage in such cases.
Merge branch 'install-fixes' into develop