created factorial function and added additional tests

[emtempleton]

No description provided.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0f001e7). Click here to learn what that means.
The diff coverage is 77.77%.

@@            Coverage Diff            @@
##             master       #8   +/-   ##
=========================================
  Coverage          ?   73.33%           
=========================================
  Files             ?        1           
  Lines             ?       15           
  Branches          ?        2           
=========================================
  Hits              ?       11           
  Misses            ?        3           
  Partials          ?        1

Impacted Files	Coverage Δ
factorial.py	73.33% <77.77%> (ø)

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[emtempleton]

passed Python 3.5 so considering this a success!

[yarikoptic]

Yeap, congrats! ;-)