Implemented plugin system, introduced experimental hooks system (three hooks defined at the moment), moved thresholding to plugin, new stack plugin for stacking charts, refactoring of data processing to support plugin writing, moved series specific global options, changed semantics of arguments to plothover event to reflect the situation with data transformations

git-svn-id: https://flot.googlecode.com/svn/trunk@157 1e0a6537-2640-0410-bfb7-f154510ff394