Amber REPL Enhancements

I have put some new features into the REPL to make it more comfortable to work with.
More will follow, but for now this is what I managed to do:

* REPL **welcome message** stating Amber and Node versions
* support for **variables**
    * `date := Date new`
* **remember** previous expression **results**
    * i.e. when you just type an expression it will be assigned a variable `res1` automatically (counting up)
* **:q to quit** the REPL
    * AFAIK you had to exit it through Ctrl+C
* **Ctrl + L** to clear screen
    * only tested on Mac OSX

I'm planning on adding more, most importantly the ability to define methods, which is actually pretty easy and straightforward compared to variables.

Here's a screenshot showing some of the features:
![amber-repl](https://f.cloud.github.com/assets/158871/694596/ae84f43a-dcb4-11e2-9696-e244054c320c.png)