Allow editing and flattening of text form fields within PDF documents

Hi all,

I'm submitting this patch to the Zend_Pdf component which adds some critical (for me) abilities:

- 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.

Whilst I understand that ZF2 is now the focus, there doesn't yet appear to be a Zend_Pdf component for ZF2 so I am submitting it here instead.

I haven't written any tests to prove out this functionality (happy to, if some guidance can be given) - but the modified code has been live in production for several months.

Thanks
Steve