Added additional version support to ebuild.install

Version can now contain <, <=, >, >=.
ebuild.install should also be able to support 'slots' being specified in
a state when using the pkgs syntax. To differentiate between versions
and slots, a slot must start with a ':' in the state.
Updated docs in pkg state to reflect new options

ebuild now supports comparisions for versions and slots
Allow Gentoo to use comparisions in check_desired
Fixed quoting pkg atoms if comparision is provided
Removed doc about slots when using pkgs with pkg.installed

Although the state would install the package correctly, it failed
to detect the correct slot as being installed and would always reinstall
it. The only solution I currently see is to put os specific code in the
state to make slots work.
Merge pull request #3531 from shadowfax-chc/gentoo-pkg-improv

Gentoo pkg improv