Added github whitelist authenticator.

[stefanoborini]

Github authenticator that performs whitelist evaluation
against a textfile for the appropriate usernames

[codecov-io]

Codecov Report

Merging #351 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage    94.9%   95.17%   +0.26%     
==========================================
  Files          88       92       +4     
  Lines        3789     3873      +84     
  Branches      247      250       +3     
==========================================
+ Hits         3596     3686      +90     
+ Misses        133      127       -6     
  Partials       60       60

Impacted Files	Coverage Δ
...eappmanager/jupyterhub/auth/world_authenticator.py	100% <ø> (ø)
.../jupyterhub/auth/github_whitelist_authenticator.py	100% <100%> (ø)
remoteappmanager/jupyterhub/auth/init.py	100% <100%> (ø)
.../auth/tests/test_github_whitelist_authenticator.py	100% <100%> (ø)
.../jupyterhub/auth/tests/test_world_authenticator.py	100% <100%> (ø)

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