Sizzle free

`:input` is only available with jQuery builds that use Sizzle. Using an equivalent CSS selector string (`input, select, textarea, button`) makes pickadate compatible with more codebases.

This particular update only adds 1 character to lib/picker.js while simplifying an event target test in `prepareElementRoot()` 
`[contenteditable]` might be a good option, although it's a little edge-case and might introduce awkward bugs - it might be worth keeping the constraint on standard `:input` targets (jQuery just does a tagName regex match for it's `:input` pseudo-selector).

`tabindex` is probably redundant if we're matching on all other input-y things?