Added PKCE support#46

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almirbi wants to merge 3 commits intomainfrom

Commits on Sep 5, 2017

Added PKCE support
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Commits on Sep 16, 2017

Refactoring; Throw an exception if unsupported challenge method;
almirbi
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Commits on Sep 17, 2017

Review changes; Add force PKCE as an option;
almirbi
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