Bugfix/grouped timeseries hover states

Fixed the issue where missing data on the left hand side of a graph caused odd parts of the graph to become highlighted when mousing over.

![](https://cloud.githubusercontent.com/assets/78156/2706502/77a76c9c-c492-11e3-9afb-e0e63d2bfc82.png)

Also involved some major refactor of `Line#getDistanceAndClosestModel` to remove the entirely un-used `allowMissingData` option.