Replaced pyPdf with PyPDF2.

This change is necessary because there are problems with installing pyPdf from
PyPI.

When I run `pip install pypdf` using pip 1.5.dev1 bundled within virtualenv’s master branch, I get the following error:

```
  Could not find any downloads that satisfy the requirement pypdf
  Some externally hosted files were ignored (use --allow-external pypdf to allow).
Cleaning up...
```

When I run `pip install pypdf --allow-external pypdf --allow-unverified pypdf`, I get:

```
ConnectionError: HTTPConnectionPool(host='stompstompstomp.com', port=80): Max retries exceeded with url: /pyPdf/ (Caused by <class 'socket.gaierror'>: [Errno -2] Name or service not known)
```

Replacing pyPdf with PyPDF2 seems to solve the problem (i.e. the tests pass).