Added support for DataSet to Graph. Graph now uses an id based set of nodes
and edges instead of a row based array internally. Methods getSelection and
setSelection of Graph now accept a list with ids instead of rows.
Graph is now robust against edges pointing to non-existing nodes, which
can occur easily while dynamically adding/removing nodes and edges.
Implemented basic support for groups in the Timeline.
Added documentation on DataSet and DataView.
Fixed selection of nodes in a Graph when the containing web page is scrolled.
Improved date conversion.
Renamed DataSet option fieldTypes to convert.
Renamed function vis.util.cast to vis.util.convert.