Updated the Gentoo Dependencies

I updated base.yaml and python.yaml with Gentoo dependencies, some existing, some on their way through:
https://bugs.gentoo.org/show_bug.cgi?id=510908

I ran nosetests with no errors and only warnings that existed before my edits. I intend to write an updated Gentoo installation guide in the roswiki when I get more important patches in.

Things that still need to be done/packaged for Gentoo:

```
# the following modules are in the rosinstall src...but I haven't figured out
# how to fix the PYTHONPATH variable to find them when running catkin_make_isolated.
genmsg
gencpp
genpy
genlisp

# do I need to package these?
catkin
rosfind

# these simply don't have packages in the portage tree yet, and I'll write ebuilds soon.
collada-dom
orocos_kld
```