add cusom name support

Good catch. <code>init()</code> is now only defining the <code>text</code> tag in the main chart. I've moved the setting of the <code>x</code> and <code>y</code> into <code>redraw()</code> along with the label text. <code>x</code> and <code>y</code> are using <code>width</code> and <code>height</code> instead of calculating <code>(getCurrentWidth() - margin.left - margin.right)</code> as before. Indentation corrected as well, so Travis CI shouldn't error out on that either.

styling changes Re: #275

Here are the styling changes moved to c3.css regarding #275

Code to place the chosen __data_empty_label in the center of the main chart when 0 targets supplied. Takes into account sub chart being active.
Styling has been moved to c3.css

I noticed that the mouseover effect of expanding and contracting was not happening with the arc gauge. I thought it was working before, and I don't know if it was taken out on purpose of left out during the refactoring of the arc portion of the code. Here is a fix that goes with the current code.