Make primitive lookup case insensitive

[kmax12]

With this PR, we now handle the case where the name of the primitive isn't already all lower case

[codecov-io]

Codecov Report

Merging #213 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
+ Coverage   93.46%   93.49%   +0.02%     
==========================================
  Files          71       71              
  Lines        7784     7789       +5     
==========================================
+ Hits         7275     7282       +7     
+ Misses        509      507       -2

Impacted Files	Coverage Δ
featuretools/primitives/utils.py	85.36% <100%> (ø)	⬆️
...ols/tests/dfs_tests/test_deep_feature_synthesis.py	98.36% <100%> (+0.02%)	⬆️
featuretools/synthesis/deep_feature_synthesis.py	93.18% <0%> (+0.56%)	⬆️

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[rwedge]

Looks good!