make installkey more flexible
allow installed action to list specific keys
expand cli output cases
create GKEY_CHECKS, a namedtuple used in gkey checks
make list-keys more flexible
add support for gkey checks

Features:
    * Enable colons listing
    * Checks for expired, invalid and revoked keys
    * Checks for keys capabilities
Merge pull request #22 from gentoo/key_checks

add support for key health checking to installed keys; general improvements