add gkey-gen implementation (GLEP 63 openPGP key generator)

 Developers and users are now able to create GLEP 63 based GPG keys
Merge pull request #18 from gentoo/gkeygen

gkey-gen: GLEP 63 openPGP key generator
add file verification support

File verification support allows users to verify files by either URLs or local files.
These files are verified against their signature and the installed release keys.
Merge pull request #21 from gentoo/file_verify

add file verification support
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