Tooltip content

Pull request includes following changes:
- precision in x or y value https://github.com/krzysu/flot.tooltip/issues/50
  - concerning $ escaping https://github.com/krzysu/flot.tooltip/issues/16
- use custom ticks if given https://github.com/krzysu/flot.tooltip/issues/18
- remove %s if series label is undefined https://github.com/krzysu/flot.tooltip/issues/41

I've also added two examples showing $ escaping and custom ticks usage.