Helpers and different color set formats

This implements what was [discussed](https://github.com/amdt/vim-niji/pull/7#issuecomment-19491497) in #7.  

Other than a few variable name changes (`g:niji_colours` over `g:niji_always_use` and `light` over `light_colours` in the maps), there's 1 difference (that I can see).  Colorscheme color sets (for instance `g:niji_solarized_colours`) take precedence over the globally defined user colour sets (`g:niji_colours`) because otherwise you'd have to remove/comment out the global one to use the more specific ones.

Ready for discussion :smile_cat: