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.
Make changes as commented by @zenden2k

I've updated the check here to be more cautious; no harm in checking to see if the fields exist before trying to use them!
Fixed syntax error
Added Unit Tests for Zend_Pdf

Commit Includes:
- PdfTest.php containing unit tests for some methods of Zend_Pdf
- Integration of PdfTest.php in AllTests.php
- PdfWithFields.pdf sample PDF file to assert functionality related to form fields

Tests include unit tests for following methods:
- `Zend_Pdf::getTextFieldNames()`
- `Zend_Pdf::setTextField()`
- `Zend_Pdf::setTextFieldProperties()`
- `Zend_Pdf::markTextFieldAsReadOnly()`
Exception assertions changed to PHPUnit 3.7+ standards
Constant `__DIR__` replaced with `dirname(__FILE__)` as `__DIR__` is not supported in PHP versions older than 5.3