Support for associations and collections filled with multiple result sets

Hi.

This change adds support for filling associations and collections out of multiple result sets. (see issue #512 in gcode)

Suppose we have the tipical N+1 problem: An order that has order lines. 

Right now MyBatis provides two ways of solving this.
- A join (nested result map)
- A nested select

Both work but have their drawbacks.

The join gets more data than needed and MyBatis should do an extra work to split the data back into entities.

The nested select is slow and resource consuming. It is much better to hit once the DB instead of N+1 times.

The proposal is to add a new way to solve this.

First, two sentences are sent to the database at the same time. 

```xml
<select id="getOrders" resultMap="orderResult">
	select * from mbtest.order_header ;
	select * from mbtest.order_detail
</select>
```

Which causes an execution like this one:

```
DEBUG [main] - ==>  Preparing: select * from mbtest.order_header ; select * from mbtest.order_detail 
DEBUG [main] - ==> Parameters: 
TRACE [main] - <==    Columns: order_id, cust_name
TRACE [main] - <==        Row: 1, Fred
TRACE [main] - <==        Row: 2, Barney
TRACE [main] - <==    Columns: order_id, line_number, quantity, item_description
TRACE [main] - <==        Row: 1, 1, 1, Pen
TRACE [main] - <==        Row: 1, 2, 3, Pencil
TRACE [main] - <==        Row: 1, 3, 2, Notepad
TRACE [main] - <==        Row: 2, 1, 1, Compass
TRACE [main] - <==        Row: 2, 2, 1, Protractor
TRACE [main] - <==        Row: 2, 3, 2, Pencil
DEBUG [main] - Resetting autocommit to true on JDBC Connection [org.postgresql.jdbc3.Jdbc3Connection@7f1ee770]
DEBUG [main] - Closing JDBC Connection [org.postgresql.jdbc3.Jdbc3Connection@7f1ee770]```

In the mappings, we include a new attribute to indicate that the data comes in the resultset with index 1 (the second)

```xml
<resultMap type="order" id="orderResult">
	<id column="order_id" property="orderId" />
	<result column="cust_name" property="customerName" />
	<collection property="detailLines" column="order_id" foreingKey="order_id" resultMap="orderDetail" resultSetIndex="1" />
</resultMap>
```

Probably this method will be the fastest of the three. 

What do you think? Should we do the change?