add support for event log API

[anneFly]

This PR adds support for the event log API. The testing stub will not record event logs by default but I added a context manager that can be used to record logs while testing so that the get_event_logs method of the testing stub will return useful data. However, for now only task creation and deletion is supported as possible event logs.

[codecov[bot]]

Codecov Report

Merging #135 into master will increase coverage by 12.65%. The diff coverage is 86.88%.

@@             Coverage Diff             @@
##           master     #135       +/-   ##
===========================================
+ Coverage   41.19%   53.84%   +12.65%     
===========================================
  Files           9        9               
  Lines         823      884       +61     
===========================================
+ Hits          339      476      +137     
+ Misses        484      408       -76

Impacted Files	Coverage Δ
closeio/closeio.py	62.63% <100%> (+0.4%)	:arrow_up:
closeio/contrib/testing_stub.py	31.41% <83.72%> (+31.41%)	:arrow_up:
closeio/utils.py	76.64% <93.33%> (+2.05%)	:arrow_up:

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[anneFly]

@anapaulagomes after you asked me why we don't generate event logs automatically in the stub, I was thinking about it. I still like the idea that the user has the option, but it made me question my implementation and I think I found a much nicer and simpler way now. It's still using a context manager to optionally record logs, but now it will record logs for all operations inside the context manager in the correct order. And also the code is way simpler now. Good that we talked about it :) I also added a test for the pagination function as you suggested. Please have a look at the last two commits once more after travis is green.

[anapaulagomes]

Good to go! 👍