added darker/brighter as alias for darken/brighten to avoid confusion
added darker/brighter as alias for darken/brighten to avoid confusion

GerHobbelt/chroma.js,rymohr/chroma.js,Lucas-naceri/chroma.js,asbien/chroma.js,gamedevforks/chroma.js,mcanthony/chroma.js,gamedevforks/chroma.js,mcanthony/chroma.js,asbien/chroma.js,Lucas-naceri/chroma.js,rymohr/chroma.js