Add support for client side certificates, closes #632

[joaompinto]

Add "cert" parameter to the Jenkins class init() that is passed down to requests library call arguments.

Reference: http://docs.python-requests.org/en/master/api/

cert -- (optional) if String, path to ssl client cert file (.pem). If Tuple, ('cert', 'key') pair.

[codecov[bot]]

Codecov Report

Merging #633 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #633      +/-   ##
=========================================
+ Coverage   78.68%   78.7%   +0.01%     
=========================================
  Files          34      34              
  Lines        2557    2559       +2     
=========================================
+ Hits         2012    2014       +2     
  Misses        545     545