Added poly term to model formulas - issue #60.

As John suggested, I implemented the `poly` term to test the waters before trying to add the spline `bs` term.  The usage is

```
d = DataFrame()
d["y"] = [1:4]
d["x1"] = [5:8]
d["x2"] = [9:12]
d["x3"] = [13:16]
d["x4"] = [17:20]
model_matrix(:(y ~ x2 + poly(x1, 3)), d)
```

Please let me know if this needs any revisions.