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()` 
@mmcgahan what about `[contenteditable]` or anything with a `tabindex`? We should probably add that in as well..?