Finish if we are in a term lacking 256 colors

The colorscheme is written only for gui and 256 color terms.  This
series of changes aims to remove the term-checking logic by collecting
gui and term colors and attributes into single highlight declarations
Eliminate all but Kurt zone t_Co if statement

* Move all term highlights from Kurt zone into the topmost highlights
* Eliminate the two if statements from the Visual section
* Do the same for the Folded/TabLine section
Collect Error, Include, and Label groups
Collect highlights from Kurt zone

* Spelling groups go up
* LineNr and MatchParen to high_contrast
* Pmenu to default
* Groups in the inner high_contrast if statement go to their
  corresponding group in the below high_contrast conditional
* Remove some groups (the ones not camel cased) that repeated previous
  highlights
Collect and rearrange

* Move force_dark_background statement below the Normal highlights
* Remove non-HC (high contrast) errormsg highlight, seems inconsistent
* Collect linenr and statusline into the big if statement non-HC part
* Collect "tabline and fold column" into the big HC if statement
Align and sort
Add highlights

to Pmenu, MatchParen, SignColumn, and TabLineFill:

* PmenuSelBar and PmenuThumb happen to be the same, move them up
* Move out the common parts of SignColumn
* Add terminal highlights for PmenuSel and SignColumn
* Make TabLineFill consistent
Adjust Visual term bg

Add CursorLineNr as a combination of LineNr and NonText
Avoid using the user's term colors

Substitute remaining occurences of colors 0-15 with hopefully the
closest xterm color to the corresponding gui color.
Merge pull request #10 from holomorph/collect-defns

Original comments from holomorph:
"This is a refactor which gets rid of a lot of logic and lines by combining the gui and term highlights. There are very few visible changes to the theme, and these changes are made only in the interest of making the colorscheme more complete or consistent between gui and term.

With these changes (the gui and term colors for a syntax group being on the same line), it is much easier to spot missing/inconsistent pieces of the colorscheme."

IMO the CursorLineNr is now too dim, but since this set of patches is just great, a new patch will be made for CursorLineNr a bit later. Thanks a lot!