Social buttons added
Social buttons styling
Twitter button url fixed
Reference data added for pb
Reference year selection
Added refrence values for all levels
Updated index links
PBItem sums for reference years added in json response
BUGBu sums for reference years added in json response
BUGBuItem sums for reference years added in json response
PEP8 length fix
Fixed bug in PBItem reference values calculation
Merge pull request #34 from jank0s/master

Social widgets added, reference years comparison implemented, prepared json resposnes for reference year visualisation