Forms validation refactoring

[IgorMinar]

This is a meta issue for refactoring of forms validation in AngularJS.

We are considering to do this in 1.3, but it might slip.

Related/Sub issues:

• when ng-maxlength is invalid, 'require' validation is also invalid #6304 - when ng-maxlength is invalid, 'require' validation is also invalid
• Input number with required validation is broken on chrome #6818 - Input number with required validation is broken on chrome
• ngOptions : regression 1.2.15 && 1.3.0-beta.3 #6828 - ngOptions : regression 1.2.15 && 1.3.0-beta.3
• fix(input): ngMinlength/ngMaxlength should not invalidate non-stringlike values #6002 - Forms validation refactoring
• Forms validation refactoring #6928 - ngMin doesn't render values that are initially invalid

[Narretz]

I jotted down some thoughts about parsers / formatters and validation here: #6740