Added support for character set on columns using MySQL adapter 

Hello, 

I couldn't set the character set on columns using MySQL adapter, so have added to the options array...

To set character set on a column add 'character' key value pair to the
$options array passed to add_column or change column.

Eg. $this->add_column('table', 'column', 'string',
array('character'=>'latin1'));

MySQL 5.0 documentation for setting column character set can be found
at http://dev.mysql.com/doc/refman/5.0/en/charset-column.html

MySQL 5.0 documentation of character sets and their default collations
can be found at
http://dev.mysql.com/doc/refman/5.0/en/charset-mysql.html

Thanks,
SB