Pyshorteners

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* add import shortcut

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* tests running now with pytest
* fix some py3 issues
* Remove GenericExpander in favor of new BaseShortener class
* All shorteners now must inherit from BaseShortener

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* Goo.gl now needs an `api_key` in order to short and expand urls
* Bit.ly now needs an extra `bitly_token` in order to short and expand urls

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* Ow.ly shortener added
* Readability shortener added

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* Add GenericExpander

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