CZML Vectors

This is a companion pull request to AnalyticalGraphicsInc/czml-writer#50, which defines a spec for vectors in CZML.

Adds `CzmlDirection`, `DynamicVector`, and `DynamicVectorVisualizer` objects with matching specs.  I don't have any cool demos yet, but I'm working on it.

CC @mrmattf @emackey