__data_empty_label when 0 targets to chart.

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
Merge branch 'patch-3' of https://github.com/bjlbernal/c3 into bjlbernal-patch-3
Suggested changes made

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.
Merge branch 'patch-3' of https://github.com/bjlbernal/c3 into bjlbernal-patch-3
Fix empty text when no data to show - #275
Fix conflict