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).
I don't think I need that.