Make LsbDetect check upstream LSB data as well.

First stab at solution for ros-infrastructure/rosdep#272.

Some slight code duplication (copy/paste from `lsb_release`), but it doesn't introduce any new dependencies.

From some googling I understand that `lsb_release` is going to be deprecated (or already is), and `platform.linux_distribution(..)` is transitioning to `/etc/os-release`. iiuc, `platform.linux_distribution(..)` is also (going to be) deprecated.

All tests pass, and my system is correctly identified as:
```
OS Name:     ubuntu
OS Version:  12.04
OS Codename: precise
```