Separate graphlot composer from generic graphlot

this is a work in progress, and made public to gather feedback. not meant for merging yet!

the goal is to separate the current jquery.graphite.js into two parts:
the graphlot code itself for drawing the graph, showing the tooltips, with the legend and zoom support.
the graphlot composer into a separate object, which binds all the UI widgets to the graphlot, so that the graphlot composer functionality from before remains unchanged, but we gain a generic, reusable graphlot object which can be used to draw interactive graphs without all the composing and modification features that the composer has.
cc @luciotorre (original author of jquery.graphite.js)
cc @xkilian who expressed some interest
contributions to this PR are welcome by forking and doing a PR in my repo for this branch

replaces #69