Update solarized colors

Updated to more closely match solarized colors.

Also updated the logic for detecting how many colors a terminal
supports.

The only thing I'm still unsure of is the difference between the 'HEX'
and 'XTERM/HEX' columns on the 'The Values' section of the solarized 
page:
http://ethanschoonover.com/solarized#the-values

Fixes #10 (as long as the [l:solarized_guifg_colours](https://github.com/joshuarh/vim-niji/blob/aa910601f12332e006941122fcb623b27920da5c/autoload/niji.vim#L85-L90) colors are correct (see above, should they be the HEX colors or the XTERM/HEX colors?))