Bump version to 2.0.0.

Allow custom email classes that extend `CakeEmail`.

Using the `Email.classname` configure key, it is now possible to
use a custom email class as long as it extends `CakeEmail`.

For example, you might have an `AppEmail` class you need to use,
just add the following to `bootstrap.php` and you're done:

    Configure::write('Email.classname', 'AppEmail');

Update Monolog to 1.7

Merge pull request #9 from burriko/monolog1.6

Update Monolog to 1.6