Related to Versus VS-109

The Admin page allows user to create an index be selecting adapter, extractor, and measure from drop-down lists. Should have a pop-up window appear as user hovers a mouse above an extractor name. Pop-up window should have some information about an extractor.

Also should make appropriate changes in Versus code to have additional method for each extractor - call it Information or Explanation (but it's NOT the descriptor/feature, so one should be careful in choosing method name to avoid confusion).

The reason for the proposed change is that currently, there are two extractors in versus-image that seem to be accepting RGBPixels, but really want a specific type of image. So displaying this information on Medici side when user selects an extractor to use would help choose correct extractor for given image.

RGBHistogramExtractor
expects pixels to have length =3. Throws exception otherwise. (In other words, expects a color image and throws exception in case of gray-scale image).
GrayscaleHistogramExtractor
Expects pixels to have length =1, throws exception otherwise (In other words, expects a gray-scale image and thrown exception in case of color image)