Make TestCase::assertSessionHasErrors() more useful

Currently, ` TestCase::assertSessionHasErrors() ` checks only that the session has the ` 'errors' ` item bound.

This change will allow that and some more specific things, like:

```php

$this->assertSessionHasErrors('email');

$this->assertSessionHasErrors(array('email', 'password'));

$this->assertSessionHasErrors(array(
    'email' => Lang::get('validation.required', array('attribute' => 'email')),
    'password' => Lang::get('validation.required', array('attribute' => 'password')),
), ':message'); // This last bit's a little weird, but it keeps the message from being wrapped in the Bootstrap span

```

I like them. Anybody else?