PDF.js refresh

This change updates PDF.js to the latest version (tip as of 3/12/2014), and changes the way PDF.js is integrated to simplify future updates. Changes to the architecture include the following:

- PDF.js is now cloned directly from the Mozilla repo (rather than our own with Mozilla as an upstream repo).
- Rather than being reconstructed by GWT UiBinders and script injection, the PDF.js application and its dependencies are now served as static resources from the session resources directory, using a new request handler for `/pdf_js`. 
- Coupling to specific elements and methods in the PDF.js viewer (which are subject to change) is encapsulated in a new class, `PdfJsWindow`. This class also injects the appropriate event handlers and modifies the UI slightly to suit the RStudio use case.
- The `PDFViewer` class now serves as the interface between PDF.js and RStudio proper, and is responsible for maintaining the state of the viewer and handling events from both the viewer window and GWT.