refactor(datepicker): track buttons by date instead of $index
refactor(pagination): move boundary & directions to the template

Closes #795
Closes #1770
feat(alert): add WAI-ARIA markup

Closes #1806
chore(demo): fix fork button width

Closes #1805
fix(datepicker): `Today` button should not set time

Fixes #1726
Closes #1808
feat(button): Allow radio button uncheck

btn-radio with uncheck attribute acts as a hybrid checkbox/radio-button, selecting exclusively among the button set, but also allowing the selected item to be unselected, leaving the button set without a selected item.
docs(button): Document btn-radio-uncheck

Add uncheckable behavior description to readme. Add uncheck example alongside btn-radio example.
docs(button) Render explicit `null`