Update PDFjs 

Update PDFjs version to d45d7bc16128e3783ad29d16f3315add8f224265.

Includes new mandatory dependencies from the PDFjs viewer, for enabling the text layer:
- `pdf_find_bar.js`
- `pdf_find_controller.js`
- `ui_utils.js`
- `text_layer_builder.js`

Removes the customized `TextLayerBuilder.js`.

Adds a few convenience methods for better inclusion of external scripts.
Should fix issue #13.