Work on the configuration system

Here is my work on the configuration system.  See the commit messages for more
details.  This is not finished yet:

- [x] New internal configuration system
- [ ] `conda config` command to modify and deal with `rc` files (see
https://github.com/continuumio/conda/wiki/config for my thoughts on how it
should be done).
- [ ] Some tests for the internal configuration system