Pojo support for Parquet.

Follow on to this pull from the previous project: 

https://github.com/Parquet/parquet-mr/pull/325

Has some additional tests and addresses some code review comments.
will a given array converter ever be used for different types? It seems like not, since component type is fixed, right? So this could have a type parameter that is the type of array. IE currentArray() could return T, and that would be boolean[], String[], etc, and you wouldn't need the casts. Might be tricky to plumb through though.