Clarify information about which versions of Ubuntu/Debian are supported

This information was implicitly in the dropdown boxes for configuration already,
but for those that don't get that far...
Add mention of Fedora 20 to the RedHat download page

Per note from Devrim. Also add a short sentence about distros
not mentioned on the page.
Fix installation instructions on Fedora 19+
Update RedHat family installation instructions

Consistently refer to "RedHat family" instead of listing individual
distributions. Include explicit mention of Oracle Linux, and which
versions it ships with by default.

Clarify exactly which distribtutions are supported by the yum
repository.