Include form Validation directive

This directive will validate the form based upon the score attribute and return invalid if the password strength is too low.

Score can be set directly in the attribute or as a scope variable.

Usage example:
```html
<form name="form">
   <input type='password' name='password' ng-model='password' zxcvbn-score score='score'></input>
</form>
<small ng-show="form.password.$error.zxcvbn">The password is too weak!</small>
```

Some other changes I made:
1. Updated example to correct module name
2. Changed the zxcvbn directive into zxcvbnCrackTime

Still TBD:
1. Include new tests and update old ones to reflect directive name
2. Update README