Add CherryPy dependency and minimal install info to doc/install.txt.

Add CherryPy dependency and minimal install info to doc/install.txt.

I seem to have left CherryPy out of the earlier list.

The extra handholding for minimal installations is an attempt to
address problems people have had installing BE on non-Linux systems,
where the standard build tools are less common.

I also mention my Gentoo overlay and Michel Alexandre Salim's Fedora
package, so people running Gentoo and Fedora don't have to bother
figuring any of this out ;).

Finally, I moved the package information up to the top of the install
page, so people on supported systems realize they have an easy out
before reading through the install procedure.