Fix legendentry for Barchart and Scatter (#182)

[johannes-fischer]

This fixes #182:

• BarChart was missing the plotLegend call.
• An array of legend entries can now be provided at once, this should make the scatter example from the docs working.

[codecov-commenter]

Codecov Report

Merging #183 (35098e4) into master (7207ce9) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   63.36%   63.34%   -0.03%     
==========================================
  Files           5        5              
  Lines         797      802       +5     
==========================================
+ Hits          505      508       +3     
- Misses        292      294       +2     

Flag	Coverage Δ
unittests	63.34% <50.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/PGFPlots.jl	54.50% <50.00%> (-0.04%)	⬇️
src/plots.jl	85.90% <0.00%> (+0.06%)	⬆️

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[mykelk]

Thanks so much!