Added the --skip-keys option.

The --skip-keys option allows to temporarily skip specific rosdep keys from the
command line. Works for the 'install' and 'check' verbs.
use custom mechanism for --skip-keys
Merge pull request #1 from ros-infrastructure/NikolausDemmel-ignore-patch

use custom mechanism for --skip-keys
Minor fixes for the --skip-keys option.
Merge pull request #301 from NikolausDemmel/ignore-patch

Added the --ignore-deps option to specify ignored rosdep keys from the command line.