Fixes #374

Changes:
- Implementation of `setJavaScript()` and `getJavaScript()` added for manipulating document-level JavaScript.
- Additionally, `resetJavaScript()` and `addJavaScript()` methods added.

See the docblocks for all four methods for the description of their operations.

#374 fixed.
I had an illusion that Zend_Pdf has methods like `Javascript` and not `JavaScript`. I must have mixed it up with another library. I'll fix that asap.