Allow editing and flattening of text form fields within PDF documents

Adds the ability to set the value of text form fields, by using setTextField($name, $value).

Also allows those fields to be made read-only, by using markTextFieldAsReadOnly($name).

This builds on code found on StackOverflow (http://stackoverflow.com/questions/9139787/how-to-fill-pdf-form-in-php/9140911#9140911) which was previously submitted as a patch, but was never accepted formally or merged in, probably due to the focus shifting towards ZF2.