Improved and extended parsing API.
Improved and extended parsing API.

* Reinstated parsing with default values, in part to support previous
  API.
* Added functions for detecting parseable values.
* Added parsing for None and empty / whitespace.
* Updated `literal` and `parse` to return (value, success) pairs which
  are needed to distinguish None or any other value that can be a valid
  parse result.
* Moved copyright notice out of documentation.

afbarnard/barnapy