#56: Add support for Python 3.
* Migrated command line tool `cutplace` to Python 3. Some rarely used functions and options are gone, in particular:
  * Validation results errors are communicated using simple iterators and `Exception`s instead of a needlessly complex observer pattern (no more `ValidationListener`).
* Removed several now obsolete modules that only add spurious complexity:
   * `interface` is now mostly part of `cid`.
   * most functions of `parsers` can be found in `_tools`.
   * `version` information is available from `_version` and `cutplace.__init__`.
Cleaned up trove categories concerning:
* #56: Add support for Python 3.
* #61: Add __future__ imports for Python 2.
* #74: Cleanup project structure and build process.
Removed broken and probably soon obsolete test case for web interface.

Related to:
* #10: Add test case for web validation.
* #56: Add support for Python 3.
* #77: Improve graphical user interface.