Use functions for storing OS key types

This allows the OS to be more flexible in determining the key type.
Use version instead of codename as key for f21+
remove lucid from the stdeb2.cfg file
remove python 2.6 override
Implement --as-root option.

 - The option allows to overwrite defaults for using sudo with specific
   installers from the command line.
 - The only installers currently not using sudo by default are homebrew and cygwin.
0.10.31
unify stdeb2 / stdeb3 config files
fix proxy code for Python 3
move imports to beginning of the file
add a test for proxy detection
Merge pull request #351 from ros-infrastructure/proxy_py3

fix proxy code for Python 3
0.10.32
ignore nose folders
have travis test python 3.2
relax dependency on python 3.3 in stdeb conf
[style] cleanup white space and make PEP8
support py3.2, conditionally use unicode literals
Merge pull request #352 from ros-infrastructure/python3.2_support

Python3.2 support
allow non-existing system wide sources.list.d

Since #342 we have the option to override the source lists via ROSDEP_SOURCE_PATH.
This fix make it possible to use rosdep without any system wide sources.list.d (e.g. if you don't have sudo).

If ROSDEP_SOURCE_PATH is not set, nothing changes.
If it is set, the first directory in that list that exists is used (with fallback to system wide one in /etc).
Merge pull request #354 from flixr/init_sources

allow non-existing system wide sources.list.d