crypto-square: Reimplement according to updated README

As pointed out in https://github.com/exercism/x-common/pull/28
the tests used to require output chunked into groups of 5 characters.

This commit changes the test suite and example implementation to
alleviate this.

* The encode function in the example implementation is rewritten to
  behave according to the new requirements.

* The decode function is removed because there is no mention of it in
  the README and because it cannot completely restore the original
  string.

* The test suite is changed to contain several test cases of increasing
  difficulty.

Fixes https://github.com/exercism/xpython/issues/130