Add a MessageId atrribute for customizing valid/invalid message position

Since I have opened the issue, #69, I modified the code a little bit by adding a `message-id` attribute that is used to define an id name of a message element. Then, you could create the message element, which has the exact same id name. The message will be put in that element, so, you can arbitrary specify where you want to place the valid/invalid message. For example,

```html
        <form name="Form" id="form1">
                <label>Required</label>
                <input type="text"
                       name="requireWatch"
                       ng-model="form.required"
                       validator="required"
                       required-error-message="Required"
                       required-success-message="Good Required" autofocus="autofocus",
                       message-id="requireWatch"/>
                <span id="requireWatch"></span> <!-- Can be put wherever you want -->
        </form>
```