Merge branch 'release/1.5_cluster_groups' into develop
set API key for demo, not to worry about it again
added README.md
removed unused import
sonar: Avoid throwing null pointer exceptions.
simplified boolean expression
sonar: Avoid throwing null pointer exceptions.
private constructor for util
missing curly braces
util classes final
demagicified numbers
sonar: final classes
refactoring: split method
fixed README encoding
issue 29 demo
fixed marker not shown when zoomed in/out to other region (issue 29)
added declusterification demo
updated version to 1.5.1, demo version 1.3
Merge branch 'release/1.5.1'