Revalidates arrays when they are replaced.

When an array was replaced the validator was not being updated,
it was just validating the original array.

This problem occurred especially in computed properties
that depend on arrays.
Merge pull request #72 from diogomafra/validate-array

Revalidates arrays when they are replaced.
Refactoring: make 'isValid' a computed property.

The property 'isValid' was being manually updated using observers,
this is not necessary, it's much simpler to just use a computed
property.
Merge pull request #73 from diogomafra/computed-isValid

Refactoring: make 'isValid' a computed property.
Don't emit uncatchable internal promises

Fixes #69