Generate resumableFileIdentifier based on a hash

[Vincouux]

Problem:
Using UUID for file identifier is a nice approach, but that makes the main benefits of resumable.js useless (avoid uploading same chunk twice). For example, if you stop the upload and restart, it will generate a new UUID, which will be different from the previous for the same file, and start the upload from 0.

Solution:
Hash the content of the file with a given algorithm and use it as file identifier.