Default to 16 colors for solarized theme unless g:solarized_termcolors i...

Added a check identical to solarized's [check](https://github.com/altercation/vim-colors-solarized/blob/master/colors/solarized.vim#L283). Now, 256 colors will be used only if g:solarized_termcolors is 256 (similar to the solarized color scheme). Refs #69

BTW I haven't written much VimL. Let me know if I can make any improvements