Add 'colors' object {} and '.setColor(s)' method to Helpers

Hi @mrdoob 
    This PR adds the requested 'colors' object to all relevant helpers.  Also I added optional color arguments to the initial helper constructors (where needed and intuitive).  Finally, to allow the user to directly change the colors, I added a .setColor or .setColors method to all the relevant helpers.  Changed colors are affected immediately on the next Render frame.
    As previously mentioned, I did not touch the Light helpers, because they derive their colors directly from their parent light source, and changing these helpers' colors is not really useful IMO.
The most difficult one to change in this PR was AxisHelper, because it is buffergeometry.  Learned a lot from that one! :)
    I personally will get a lot of use out of the ability to change the helpers' colors on the fly.  Hopefully others will too.  Let me know if I need to update or correct anything.  Thanks!
-Erich