Support multiple column families for GetRequest

Attached patch adds support for multiple column families in a GetRequest. Also enhances HBaseRpc.toStringWithQualifiers() to stringify such RPCs.

Here is how the modified string representation will look like

```java
public class TestGetRequestToString {
  public static void main(String[] args) {
    GetRequest get = new GetRequest("tables".getBytes(), "row1".getBytes(), 
        new byte[][] { "f".getBytes(), "g".getBytes() },
        new byte[][][] {
          {"col1".getBytes()},
          null
        }
    );
    System.out.println(get);
  }
}
```
Output:
`GetRequest(table="tables", key="row1", families=[{name="f", qualifiers=["col1"]}, {name="g"}], attempt=0, region=null)`