[Rivet] New Rivet bootstrap script

[Frank Siegert]

Dear Riveters,

As discussed heatedly all day now, the Rivet 2.x installation
bootstrap script and installation instructions needed an overhaul. I
have committed a new script and restructured the installation
instructions and would be happy about any feedback:

  https://rivet.hepforge.org/trac/wiki/GettingStarted

(Note, that there is also an undocumented and neatly separated
INSTALL_RIVETDEV option, for everybody who wants to install the
development version of Rivet. This also includes an installation of
cython, which at best can be called fragile, in particular in case you
ever want to update your Rivet working copy without running the
bootstrap script again. So if you have the chance, do install cython
with your system package manager in advance.)

Cheers,
Frank

PS: Andy, I'm copying the rivetenv.sh scripts to INSTALL_PREFIX
instead of BUILD_PREFIX now, hope that's ok?

PPS: Anybody mind if we remove
https://rivet.hepforge.org/trac/wiki/ManualInstallation and just refer
to the bootstrap script as instructions?