* Parameters with non-integer keys can no longer be created with
showkey=False, nor have the value of this attribute be set to False
* 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
* Added tests for Tag.__children__() and Tag.__showtree__() involving
attributes that have no values.