Added a candidate .clang-format file.

Please try it out, like so:
$ clang-format -i `find . -name "*.cc"`
and then look at
$ git diff

This is pretty similar to our current format,
though it fixes a few things, such as my old //commment instead of
// comment habit.

I'm open to changing the style. For instance:
- having a space after if/for/while,
- always putting method return types on their own line.
- Putting the { on the same line.