Support SQLAlchemy engines directly

This provides `discover`, `resource` and `compute(field)` access to sqlalchemy engines directly, without need for a `blaze.SQL` object (which I may soon deprecate).

```Python
In [1]: from blaze import *

In [2]: db = Data('sqlite:////home/mrocklin/workspace/blaze/blaze/examples/data/iris.db')

In [3]: db
Out[3]: Engine(sqlite:////home/mrocklin/workspace/blaze/blaze/examples/data/iris.db)

In [4]: db.dshape
Out[4]: 
dshape("""{
  iris: var * {
    sepal_length: ?float64,
    sepal_width: ?float64,
    petal_length: ?float64,
    petal_width: ?float64,
    species: ?string
    }
  }""")

In [5]: db.iris
Out[5]: 
    sepal_length  sepal_width  petal_length  petal_width      species
0            5.1          3.5           1.4          0.2  Iris-setosa
1            4.9          3.0           1.4          0.2  Iris-setosa
2            4.7          3.2           1.3          0.2  Iris-setosa
3            4.6          3.1           1.5          0.2  Iris-setosa
4            5.0          3.6           1.4          0.2  Iris-setosa
5            5.4          3.9           1.7          0.4  Iris-setosa
6            4.6          3.4           1.4          0.3  Iris-setosa
7            5.0          3.4           1.5          0.2  Iris-setosa
8            4.4          2.9           1.4          0.2  Iris-setosa
9            4.9          3.1           1.5          0.1  Iris-setosa
...

In [6]: db.iris.species.distinct()
Out[6]: 
           species
0      Iris-setosa
1  Iris-versicolor
2   Iris-virginica
```