Add DECIMAL data type support

Adds support for Sequelize.DECIMAL, which takes optional arguments for precision and scale.  Fixes #232.

You may want to change this to be a simpler function like `Sequalize.Array`.  My solution is slightly overcomplicated, but it makes this work:

```javascript
assert.equal('DECIMAL', Sequelize.DECIMAL);
assert.equal('DECIMAL(10,2)', Sequelize.DECIMAL(10, 2));
```

... which I think is quite natural.