Make attr setters in Plottable chainable

This change simply returns ``self`` from Plottable setter methods which would normally return ``None``.  This allows you to chain a set of modifier methods.

Example:

```python
hist = Hist(100, 0, 100)
hist.decorate(color='red', linewidth=2)
hist.SetFillStyle(0)
```
can become
```python
hist = Hist(100, 0, 100).decorate(color='red', linewidth=2).SetFillStyle(0)
```