Fix #12757 - use jscs to check the style guide

Right now, new task covers only <code>src</code> files, if this task will be useful, i will implement the check for the rest of them.

As i said in the letter: this tool does not cover all our code style issues (at least right now), but most of them.

/cc @rwaldron, @jzaefferer, @scottgonzalez