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.
fetch distros in alphabetic order
Use default installers from platform definitions
Remove OS_DEBIAN from gbp generation

This was removed in favor of keeping supported platforms listed in the yaml file as described in REP 137
Support RPM virtual packages

Virtual packages can be used to add dependencies on specific libraries,
make a dependency architecture specific, and much more.
Merge pull request #315 from cottsay/rpm-provides

Support RPM virtual packages
fix macports rosdep invocation
fixup circular dependency
consider test dependencies
Merge pull request #1 from ros-infrastructure/cottsay-master-fixup

fixup circular dependency
Fix installer vs installer_key
Fix command order in .travis.yml
Add rosdistro to .travis.yml as it is in setup.py
Use alternate import syntax
Merge pull request #318 from ros-infrastructure/rep140

consider test dependencies
Merge pull request #301 from NikolausDemmel/ignore-patch

Added the --ignore-deps option to specify ignored rosdep keys from the command line.
Merge pull request #302 from cottsay/master

Fedora support for catkin packages in gbprepo