#15 Add support for section column break

[bishop]

RTF spec allows for mandatory column breaks [1], and I've encountered a case where it's beneficial. One method seems all that's needed:
library/PHPRtfLite/PHPRtfLite/Container/Section.php

/**
* insert a column break
*/
public function insertColumnBreak() {
$this->writeRtfCode('\column');
}

There are other breaks in the spec, which may deserve methods as well.

[1] http://msdn.microsoft.com/en-us/library/aa140283%28v=office.10%29.aspx#rtfspec_specialchar