renamed source to maxwell

- Renamed the `source` directory to `maxwell`
- Fixed all includes to be relative to the project root, e.g. "maxwell/ast/
  Node.hpp"
- Moved the `source/thirdparty` directory to the project root
- Replaced all © with (c)

This closes #13.
fixed tests, README, and doc

Unit tests referred to old executables in `source` instead of `maxwell`. Also
fixed the README file which referred to `source`, as well as the Doxygen file
used for documentation.