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="getOrder" resultMap="orderResult">
  select * from mbtest.order_header where order_id=#{id};
  select * from mbtest.order_detail where order_id=#{id}
</select>
```

Which causes an execution like this one:

```
DEBUG [main] - ooo Using Connection [org.postgresql.jdbc3.Jdbc3Connection@f231023]
DEBUG [main] - ==>  Preparing: select * from mbtest.order_header where order_id=?; select * from mbtest.order_detail where order_id=? 
DEBUG [main] - ==> Parameters: 1(Integer), 1(Integer)
TRACE [main] - <==    Columns: order_id, cust_name
TRACE [main] - <==        Row: 1, Fred
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
```

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">
  <collection property="detailLines" resultMap="orderDetail" resultSetIndex="1"/>
</resultMap>
```

Probably this method will be the fastest of the three. 

Right now it only works for one element, so it does not solve the N+1 problem but the 1+1 problem :) But it is a point to start from.

What do you think? Should we do the change?