Add "id" to tooltip element

Was added a optional "id" in tooltip element. This model made possible to have and manipulate many tooltips in same document.
Support for stacked percent plugin

Added support for %p when using stacked percent plugin.
Merge branch 'patch-1' of https://github.com/willianganzert/flot.tooltip
Merge branch 'master' of https://github.com/larsenmtl/flot.tooltip
Essentially re-wrote Gruntfile
Cleaned and reorganized repo to act completely independent from original.
Added note to README
Replaced IE8 polyfill and usage of Array.indexOf with jQuery's $.inArray utility funciton (which provides the same functionality with a cross-browser compatibility mindset).