Highlight TODOs in slim comments

Just added the highlighting for TODOs tags in slim comments. 
- Added a keyword line (new class name : slimTodo)
- Completed the slimComments to indicate that it contains slimTodo
- And linked slimTodo to vim default Todo class 

![Before](https://f.cloud.github.com/assets/769918/285591/a4bd0a3c-9222-11e2-9c30-ea353c6f3cad.png)
![After](https://f.cloud.github.com/assets/769918/285590/a42e8334-9222-11e2-8692-e93ed46f0931.png)