SDK / Tools Updates + More Unit Tests

[halfhp]

Just some mundane maintenance stuff with a few additional unit tests.

[codecov-io]

Codecov Report

Merging #38 into master will increase coverage by 4.02%. The diff coverage is 56%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   66.36%   70.39%   +4.02%     
==========================================
  Files         104      103       -1     
  Lines        4897     4877      -20     
  Branches      518      506      -12     
==========================================
+ Hits         3250     3433     +183     
+ Misses       1313     1100     -213     
- Partials      334      344      +10

Impacted Files	Coverage Δ
...ava/com/androidplot/ui/widget/TextLabelWidget.java	76.11% <ø> (+23.17%)	:arrow_up:
.../java/com/androidplot/xy/LineAndPointRenderer.java	70.32% <ø> (+10.96%)	:arrow_up:
...src/main/java/com/androidplot/xy/StepRenderer.java	0% <ø> (ø)	:arrow_up:
...e/src/main/java/com/androidplot/xy/RectRegion.java	84.31% <ø> (+19.35%)	:arrow_up:
...-core/src/main/java/com/androidplot/xy/XYPlot.java	71.24% <100%> (+4.23%)	:arrow_up:
...main/java/com/androidplot/util/PlotStatistics.java	77.55% <100%> (+77.55%)	:arrow_up:
...dplot-core/src/main/java/com/androidplot/Plot.java	59.11% <14.28%> (+10.58%)	:arrow_up:
.../src/main/java/com/androidplot/util/LayerHash.java	51.78% <60%> (+23.52%)	:arrow_up:
...rc/main/java/com/androidplot/util/SeriesUtils.java	83.33% <66.66%> (+11.37%)	:arrow_up:
...rc/main/java/com/androidplot/ui/widget/Widget.java	66.46% <0%> (+1.19%)	:arrow_up:
... and 12 more

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