* 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.
Before, if locale wasn't specified as a parameter, it was picked from
the system running gpapi. Similar thing happened for timezone, but its
value was hardcoded in config.py.
To avoid problems related to missing or unavailable apps, this commit
enforces users to provide a locale and a timezone value. Without them,
gpapi can't be initialized.
During DeviceBuilder initialization, retrieve locale from system using
python `locale` module, and set timezone to `Europe/Berlin`. Added some
helpers to get and set those values at runtime.
