Suppress Revoked Keys in Push Command

This addresses issue #824 in keybase/keybase-issues. Revoked keys
shouldn't be considered for pushing to keybase.io as a user's key.
The commit adds a is_revoked method to the BaseKey object in
gpg-wrapper to help filter out unusable keys.
Merge pull request #179 from jaschob/master

Suppress Revoked Keys in Push Command