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* Updated user agent string with data taken from play-store-api
* *download* and *delivery* functions will now return a python generator
rather than raw bytes, to prevent app loading entire files in memory.
This generator can be iterated to get chunk of bytes to write, as
discussed in issue #35.
* as a consequence of the previous point, there is no more progress bar
feature for downloading. It should be implemented by developers using
the API.