Handle 2D Arrays

See https://code.google.com/p/vba-json/issues/detail?id=8

Example:

``` VB
Dim Test(1, 1) As Variant
Test(0, 0) = 1
Test(0, 1) = 2
Test(1, 0) = 3
Test(1, 1) = 4

Debug.Print JSONConverter.ConvertToJSON(Test)
' -> [[1,2],[3,4]]
```
