[MRG] support creation of array branches with tree models

```python
from rootpy.tree import Tree, TreeModel, FloatCol, IntCol, FloatArrayCol
from rootpy.io import root_open
from random import gauss

f = root_open("test.root", "recreate")

# define the model
class Event(TreeModel):

    x = FloatCol()
    y = FloatCol()
    z = FloatCol()
    # support for fixed-length arrays
    f = FloatArrayCol(5)
    i = IntCol()

tree = Tree("test", model=Event)

# fill the tree
for i in xrange(100):
    tree.x = gauss(.5, 1.)
    tree.y = gauss(.3, 2.)
    tree.z = gauss(13., 42.)
    for j in xrange(5):
        tree.f[j] = gauss(-2, 5)
    tree.i = i
    tree.fill()
tree.write()

f.close()
```