Check if paths may already include executable

The current convert-path-finding logic blindly appends the `convert_name` (the name of the executable) to the end of every path before checking it, but we give some examples where it is already on the end, and many users may logically think this is valid based on the setting `convert_path` (not `convert_directory`). This suppresses concatenation for path strings that already end with the executable name.
Merge pull request #62 from tdhsmith/path-contains-convert

Check if paths may already include executable
Update README to include OSX brew fix