Do not allow non-dates being parsed as dates

[jmayday]

I removed code allowing values which do not match format being parsed as dates.

[jmayday]

This is just squashed #3288

[wesleycho]

This does not look like it would be a valid fix.

I am hesitant to merge this, especially since I have a fix that will likely be merged later this week here.

[wesleycho]

After thinking about this, instead of the new Date constructor, it would be fine to have the value set to null. A test would need to be added for this that it turns invalid date strings/objects into null - this should be accomplishable by essentially doing element.controller('ngModel').$formatters[0] to grab the formatter unshifted into the array (if there isn't test code already doing so).

It also should not return undefined, but null IMO. The if-else conditional there can be refactored to then do only one return after $setValidity is called. This would also require testing.

[chrisirhc]

I'm moving this out of 0.13.0 milestone because:

1. this is a breaking change. Users might be relying on this behavior.
2. this needs tests for validity of the date, so there's still some work needed here.

[rvanbaalen]

@jmayday Please update your PR so it can be merged when the time comes.

[jmayday]

So, to wrap things up (because I'm little confused right now) - should we abandon this PR because of #3417 like @wesleycho suggested or should we keep it? (null approach also suggested by @wesleycho)?