This pull request implements `ResultTransformers` for [Guava collection types](https://github.com/google/guava/wiki/CollectionUtilitiesExplained) (`Multimap`, `Table`, `BiMap`). The [`ResultTransformers`](http://www.querydsl.com/static/querydsl/latest/reference/html/ch03s02.html) form a fantastic API to project tuples to a composition of collections, most notably `Map` and `List`. However, deep nested `Maps` of `Lists` or `Maps` of `Maps` can lead to superfluous and erroneous code. The Guava collection types help with this, for example: `Multimap` forms a fluent API around a `Map<K, Set<V>` and `Table<R, C, V>` is basically a two-dimensional `Map`. This pull request adds factory methods in `GuavaGroupBy` to construct `ResultTransformers` for these Guava collection types.
