Add gitignore file to exclude build directory
The external resource was loaded twice in Alfresco 4.2 that uses Surf checksum for dependencies.
Since the checksum is unknown, we need a way to find this file.
Resolved by
< 4.2 - Use the url as before, i.e. use the pdf.js with or without min extension depending on debug setting
4.2  - Iterate loaded external resources and find the one that matches base url (sans checksum), this is the one to use.

closes #2