Rendering bugs in exporting threat model to image

[Glavin001]

Todo:

• Currently some of the layers are rendered out of order.
  • Render each layer on their own canvas
  • merge canvas layers, see http://stackoverflow.com/questions/3750580/save-many-canvas-element-as-image/3797989#3797989
• color is a little different as the jsplumb chart and some css effect is not render normally such as the dotted border: 1px dotted gray , this may the issue of html2canvas

/cc @Pana and #8