Also removed the max cycles stop-gap, allowing much more complex pages
to be parsed quickly without losing nodes at the end

Also fixes #65, fixes #102, fixes #165, fixes #183
Also fixes #81 (Rafael Nadal parsing bug)
Also fixes #53, fixes #58, fixes #88, fixes #152 (duplicate issues)

* Parameters with non-integer keys can no longer be created with
  showkey=False, nor have the value of this attribute be set to False
  later.
* Calling Template.remove() with a Parameter object that is not part of
  the template now raises ValueError instead of doing nothing.
* Added tests for HTMLEntity._unichr() being called with out-of-range
  codepoints.
* Added tests for Tag.__children__() and Tag.__showtree__() involving
  attributes that have no values.