2 is dead, long live 3. Mainly turning print info a function
and urllib import fixes

* 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.

Patch mostly by @spointy.

Requested by Σ.

Reported by Σ.

**Breaking change**; closes #51.

Bump version to 0.4.dev.