Gulp tasks for checking and building js

Gulp tasks are added for checking js files with jslint and building target js files with gulp plugins (#222).
File package.json allows to install all requried plugins with console command 'npm install' inside project folder. To run gulp task write in console 'gulp build:js' or 'gulp jslint' inside project folder.

Also I suggest to use scss for styling graphics. With scss you can write for example:

    $graphics-colors: ['blue', 'red', 'yellow', 'green'];
    @for $i from 1 through length($graphics-colors) {
        $color: nth($graphics-colors, $i);
        .area#{$i}-color { fill: $color; }
        .line#{$i}-color { stroke: $color; }
        .line#{$i}-legend-color { color: $color; }
    }

for styling all your lines, adding style for new line would be fairly simple (just add color to array).
I prepared gulp task for building scss files into css (with minification as bonus).
It's better to use requirejs for concatenation and minification of modules anyway, so the code will be changed then.