Updated package to use go-mod compatible oauth2

[vgarvardt]

Compatibility with https://github.com/go-oauth2/oauth2/releases/tag/v4.1.0

[codecov-commenter]

Codecov Report

Merging #16 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #16   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files           4        4           
  Lines         182      182           
=======================================
  Hits          156      156           
  Misses         13       13           
  Partials       13       13           

Impacted Files	Coverage Δ
client_store.go	82.60% <ø> (ø)
token_store.go	83.92% <ø> (ø)

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