Refactor second half of HTMLBuilder.tag().
Delete 'convert_boolean_attrs()'.
Rename 'link_to()' to 'link()', etc.
Delete 'xml_declaration()'.
Merge 'test_html_attrs.py' into 'test_html.py'.
Refactor test functions into classes.
Move TestHTMLTestCase to webhelpers2.tests init.
Delete stray print.
Merge 'css_classes()' into 'HTMLBuilder.tag()'.

Some of the code was accidentally checked in earlier in d11a2ebda4 .
Merge 'test_tags2.py' into 'test_tags.py'.
Delete redundant tests.
Refactor generic test classes to HTMLTestCase.
Bugfix in 'BR' constant.
Call 'HTML.tag()' in tag helpers.
Unpack nested tag expression in 'button_to()'.
Use unicode_literals.