adding feature columns_align to set alignment for each columns, with exa...

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/2386359/1580029/34230c24-51b0-11e3-9664-805b2b930164.png)
![flexigrid-column-align-center](https://f.cloud.github.com/assets/2386359/1580031/38161326-51b0-11e3-92ce-ab0192a6cabf.png)
![datatables-column-align-right](https://f.cloud.github.com/assets/2386359/1580033/3c65154e-51b0-11e3-8683-6e9f5d703d74.png)

![datatables-column-align-center](https://f.cloud.github.com/assets/2386359/1580034/3f0daebe-51b0-11e3-87d5-a063259cd926.png)



I have add this example in examples file.