Respect Z-Orders

[dougnd]

Change to respect the z-orders of objects. The basic idea was instead of just storing the content to be written out to file in an array, maintain it in a dictionary that maps z-order to an array of content for that z-order. Then flatten it into a single content array using the z-order. This flattening does occur a couple times (for example all axes objects must be flattened so they can be book ended by \begin{axis} and \end{axis} correctly.

Before, bar charts with error bars were plotted as:

Now:

(The ugliness of these plots are to make sure there is a difference in the hashes, thin error bars weren't working)

[dougnd]

Note: fancybox phash needed to be changed because it was affected by the z-order.

Before it was rendering like:

Against a reference of:

Now it renders like:

[nschloe]

Great contribution!