Have rosdep update use new rosdistro API (REP137)

the 'gbpdistro' entries in /etc/ros/rosdep/sources.list.d/*.list
are now ignored, and replaced with data retrieved from the
rosdistro API.
Still behaves as previously on electric & fuerte gbpdistros

rosdep will output a warning, though.
Added dependency to rosdistro in setup.py and stdeb.cfg
Reverted old rep3.py behavior for old distros

Will output a warning
Revert old behavior for compatibility with old distros
Fix bug in rosdistro helper

When hot-switching index url, need to flush the cache
Added unit test, fixed another one

Also fixed a potential pitfall with editors that strip trailing whitespaces
Forgot to add the rosdistro index for the unit test
Disable REP137 deprecation warnings on the command line
Add an except case for the homebrew tap name generation

This will only be used for fuerte
For groovy and later, the behavior has been altered to using
env variable ROSDEP_HOMEBREW_TAP (defaults to 'ros' if not set)
Use private class for the cache in rosdistrohelper
Merge pull request #236 from po1/rep137

Following REP137, port rosdep to new rosdistro API