move wskflow's styling properties into a css file

[kerryspchang]

Right now, wskflow sets a node's style both in a CSS file (ibm.css) and in code (graph2dom.js). This makes debugging styling difficult (such as bug #605).

A better way to do this is to move wskflow's css properties into a wskflow.css file. In the code, set DOM attributes to use different styles. And use the DOM attributes to write tests for styling.