Add required to opt out of validation

[TheSharpieOne]

Resolves #96 by adding and documenting the required attribute.
When require is false or is a scoped variable which is false,
the form validation will be removed.
This is opt out to prevent a breaking change and usually the validation is
preferred.

This change also includes the changes to use the recaptcha expired callback as they would have conflicted if made separately.

[TheSharpieOne]

This also would succeed #73, rendering it obsolete.

Some differences between this and 73:
no use of ngRequired: ng prefix is for official angular team use. just required is used and since this is not a legit input, it doesn't fall under the "any value makes it required" problem.
required without a value: Often times, people use required without a value to indicate something is required. This change will do just that, 73 will not.
Slightly more DRY: Moved the common logic to it's own function.

[iambrosi]

I really like what you did in 4a8eb7e. You have my 👍, but I'd way for #106 to get merged before merging this one(I guess you'd have to do a rebase after merging that PR).

[iambrosi]

Thanks @TheSharpieOne !