stops pan controls when dragging outside svg

 better ux when making large drags
Double click handling for zoom in and out
removed extraneous delta power from double click zoom
Merge pull request #8 from AlexanderPico/patch-1

stops pan controls when dragging outside svg
Merge pull request #9 from AlexanderPico/master

Double click handling for zoom in and out
Adds handling of min/max zoom and onzoom to double-click
Merge pull request #11 from AlexanderPico/patch-2

Adds handling of min/max zoom and onzoom to double-click
Merge pull request #12 from ariutta/master

merge in changes from master