Support to define different OS rules in different files.

 - First file that is parsed that defines an OS resolution wins.
 - All versions of an OS must be defined in the same file (for now)
Fix file url parsing in sources_list.py

When the url scheme is 'file' netloc is empty
Merge branch 'master' into os-failover

Conflicts:
	src/rosdep2/lookup.py
Added PIP as an installer key for Fedora/RHEL
Merge pull request #207 from smd-ros-pkg/master

PIP needs to be added to Fedora rosdep
Added codename support to Fedora

I don't have access to any RHEL machines, but they may support codenames, too.
Merge pull request #208 from smd-ros-pkg/master

Codename support for Fedora
Merge pull request #26 from jamuraa/os-failover

Support to define different OS rules in different files.
Merge pull request #41 from fmanco/master

Fix file url parsing in sources_list.py
Merge pull request #53 from flixr/master

continue installing resolvable dependencies if robust option is given