Modifying dumpjson() in HTMLFileSession to be able to dump to file
Moving DataSpec and ColorSpec into properties.py, and merging functionality of MetaGlyph into MetaHasProps, and removing MetaGlyph

The root purpose of this was so that I could put ColorSpecs into the mix-in classes of LineProps and FillProps.

TextProps does not declare its color to be a ColorSpec, because in general, text properties are commonly used in places where color is not data-driven.  If data-varying color is desired for text, then explicitly redeclare the relevant text properties as ColorSpecs in the Glyph subclass itself.
Fixing a couple of bugs in generating HTML with relative paths in HTMLFileSession

First, a local assignment to the "js" variable was masking the function argument, and secondly, a logic error in the conditionals for js and css meant that calling the function with explicit keyword arguments would not override the attributes set on the session object itself.