Add panning interaction model functions to dygraph-externs

Add the startPan(), movePan(), and endPan() functions to the externs file. These functions are commonly used when defining custom interaction models and must be defined in order to satisfy the closure compiler.
Add Dygraph.PointType to dygraph-types

Add the Dygraph.PointType type to the dygraph-types externs file. This type is used when defining a clickCallback function and must be defined in order to satisfy the Google Closure compiler.
Merge pull request #305 from brentwalther/master

Add panning functions to dygraph-externs and PointType to dygraph-types