Add Accessibility Support (a11y)

Changes:

• Use aria-labelledby for input descriptions
• Make vm.submit() private
• Remove no longer relevant $.error component
• Add support of a11y validating required input
• Use <form>/<legend> to define the form title
• Add support of role="search" on <form/>
• Add support of aria-label on <form/>

Used a11y resources:

• MDN - ARIA: form role
• W3C Web Accessibility Tutorials