Added positionDynamic option
PositionDynamic : tooltip had rather follow the elProxy. Will need to be changed someday, when tooltipster is in charge of the icon positioning too
Renamed option to 'positionTracker'. Optimized it for fixed position elements
positionTracker : will also take the element size changes into account
Merge pull request #80 from iamceege/positionDynamic

Added positionTracker option to the dev branch, to allow an open tooltip follow its element if it moves.