Whitespace fixes
Reformat if statement with braces
Reduce number of string concatenations

Simplify the code by setting $alter to include $colname
PostgreSQL: AlterColumnSQL don't remove default NULL value

The preg_replace statement was doing a global replace of 'NULL' by '',
ignoring the fact that a column can be 'DEFAULT NULL', causing the
function to generate incorrect SQL.
PostgreSQL: AlterColumnSQL properly reset default for NULL
Update changelog
Merge branch 'master' into mantis-1.3