adding new feature columns_align to set alignment for displayed columns

New Feature `columns_align()`
===========================

Description : 
-----------------
This function used to set alignment for displayed columns

How to use:
----------------
`columns_align` take array as parameters. It should be `column_name` as a **key** and `alignment_type` as **value**.

Examples:
---------------
````
$crud = new grocery_CRUD();
$crud->columns('column1', 'column2', 'column3');
$crud->columns_align(array('column1' => 'center', 'column3' => 'right'));
....
````

that will produce:

column1 in center alignment
column2 in left alignment (default)
column3 in right alignment

See this:
![flexigrid-column-align-right](https://f.cloud.github.com/assets/2386359/1580266/b5a9016a-51b8-11e3-80ca-29dc71aee9e8.png)
![flexigrid-column-align-center](https://f.cloud.github.com/assets/2386359/1580267/b77d011c-51b8-11e3-823f-21a90a38f9fa.png)
![datatables-column-align-right](https://f.cloud.github.com/assets/2386359/1580268/b975053c-51b8-11e3-878d-7ecad805789f.png)
![datatables-column-align-center](https://f.cloud.github.com/assets/2386359/1580269/bb792ade-51b8-11e3-9cb8-d8d1ec432db2.png)

I have add this example in examples file.