add initial changelog with info for 3.1.0

[mattsb42-aws]

As discussed in #129 and #99, this project currently lacks a changelog. I'm putting this here as a starting point for resolving that.

[codecov]

Codecov Report

Merging #132 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #132   +/-   ##
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  Coverage   96.52%   96.52%           
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  Files          14       14           
  Lines        1065     1065           
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  Hits         1028     1028           
  Misses         37       37

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[zejn]

The changelog itself looks great, with one comment.

Looks to me that the Python "3.7" testing actually happens in Python 3.5. The testing environment itself is 3.7, but the tox command uses 3.5. See here https://travis-ci.org/mpdavis/python-jose/jobs/517511343#L237

[mattsb42-aws]

doh...that was a copy-paste fail on my part.

[mattsb42-aws]

#135 created to fix that

[zejn]

Thank you for this contribution, looks awesome now.