Fixing a bug in which, while merging two configuration trees, we encount...

...er the dict/string problem.  Fixes #65.

I kind of want to pave this over in the future by going back to dot-path configuration lookups (rather than storing as a dictionary), but that would be a fairly severe, backward-incompatible change :-/. 

Perhaps this future alternative:

1. Still allow people to access configuration values as dictionary keys, but
2. Add a method to the config object (perhaps `.get_value(dot_path)`) accepting the dot path to a configuration value that you'd like returned.

For now, though, I think this is the best way to go forward.