slt-release-finish,start: gitflow release tools

These tools implement the gitflow release process, auto-generating the
changelog and setting the npm package version.

/to @rmg @bajtos 

I needed to cut a strong-supervisor release that was not from the head of origin/master, unsupported by hubflow, so I finally replaced it with a few lines of shell scripts, as threatened.
I just pushed up defe77f41e552fdd9759dd6850bec8752c3936d5 which allows the commit message to be generated properly:
`slt-changelog --version $V --summary | git commit -F-`