Added initial fuzzers for integration with OSS-Fuzz.

[DavidKorczynski]

Cross-referencing:

• Continuous fuzzing by way of OSS-Fuzz #183
• httplib2: initial integration. google/oss-fuzz#5000

[temoto]

Note: merge at 2021-01-21 03:00 if tests pass and nobody rejects

[codecov]

Codecov Report

Merging #184 (8e4c548) into master (595e248) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   76.16%   76.16%           
=======================================
  Files           8        8           
  Lines        2618     2618           
=======================================
  Hits         1994     1994           
  Misses        624      624           

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