Fixes #17 - removes support for node.js 0.6.
Fixes `var` formatting.
Replaces 'seq' with 'async' to make the more easily express the flow.

* Seq hasn't been updated for 2 years now.
* Async offers more natural and undestandable approach.
Refactors the newly added, 'async'-based flow.
Improves formatting of imports block.
Updates assets-expander and enhance-css dependencies.

* Both at 1.0.x.
Updates less.js dependency to 1.7.x.
Updates uglify-js from 1.2.x to 2.4.x.

* Uses utility `minify` method to make code easier to understand.
* Uses different handler for `lineBreakAt` option as UglifyJS2 can output breaks after semicolons only.
Removes unnecessary test step.
Refactors all code using module pattern.