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)
Merge branch 'master' into os-failover

Conflicts:
	src/rosdep2/lookup.py
Merge pull request #26 from jamuraa/os-failover

Support to define different OS rules in different files.