Provide rootpy-like TObjArray and TList

The neat thing about this is that it can allow you to rearrange the draw order of primitives on a canvas, or remove some, like so:

```python
import rootpy.ROOT as ROOT
prims = ROOT.gPad

# Modify drawing order
prims[:] = prims[:2] + prims[-2:] + prims[2:-2]

# Remove the third plot
prims[2:3] = []
```

The above reorders the drawing of objects on the canvas so that what were previously the last two plots get moved to the front.

I've tried to ensure that object lifetimes remain correct. This really needs tests but I don't have time to write them right now.