adding unit paramater to timesince primitives (issue 503)

[ctduffy]

fixes #503

[codecov]

Codecov Report

Merging #558 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #558      +/-   ##
==========================================
+ Coverage   96.31%   96.33%   +0.02%     
==========================================
  Files         115      116       +1     
  Lines        9167     9221      +54     
==========================================
+ Hits         8829     8883      +54     
  Misses        338      338

Impacted Files	Coverage Δ
featuretools/utils/time_utils.py	95.65% <100%> (+1.2%)	⬆️
featuretools/tests/utils_tests/test_time_utils.py	95.55% <100%> (+1.26%)	⬆️
featuretools/utils/api.py	100% <100%> (ø)	⬆️
...retools/primitives/standard/transform_primitive.py	100% <100%> (ø)	⬆️
.../tests/primitive_tests/test_transform_primitive.py	100% <100%> (ø)
...ools/primitives/standard/aggregation_primitives.py	94.58% <100%> (+0.09%)	⬆️
...tools/tests/primitive_tests/test_primitive_base.py	100% <100%> (ø)	⬆️

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[rwedge]

Can we also add an args section and an example to the TimeSinceFirst docstring

[rwedge]

Looks good