Reduce the API visible through __dir__

Aggressively remove methods and properties from expressions.

What of the following list is non-essential?  What else can we remove?

```Python
In [1]: from blaze import *
df
In [2]: iris = Data('/home/mrocklin/workspace/blaze/blaze/examples/data/iris.csv')

In [3]: iris.
iris.columns       iris.fields        iris.ndim          iris.sepal_length
iris.count         iris.head          iris.nunique       iris.sepal_width
iris.count_values  iris.isidentical   iris.petal_length  iris.shape
iris.data          iris.like          iris.petal_width   iris.sort
iris.db            iris.map           iris.relabel       iris.species
iris.distinct      iris.max           iris.resources     
iris.dshape        iris.min           iris.schema        

In [3]: iris.sepal_length.
iris.sepal_length.count         iris.sepal_length.min
iris.sepal_length.count_values  iris.sepal_length.ndim
iris.sepal_length.distinct      iris.sepal_length.nunique
iris.sepal_length.dshape        iris.sepal_length.relabel
iris.sepal_length.fields        iris.sepal_length.resources
iris.sepal_length.head          iris.sepal_length.schema
iris.sepal_length.isidentical   iris.sepal_length.shape
iris.sepal_length.isnan         iris.sepal_length.sort
iris.sepal_length.label         iris.sepal_length.std
iris.sepal_length.map           iris.sepal_length.sum
iris.sepal_length.max           iris.sepal_length.var
iris.sepal_length.mean          

In [3]: iris.species.
iris.species.count         iris.species.label         iris.species.relabel
iris.species.count_values  iris.species.like          iris.species.resources
iris.species.distinct      iris.species.map           iris.species.schema
iris.species.dshape        iris.species.max           iris.species.shape
iris.species.fields        iris.species.min           iris.species.sort
iris.species.head          iris.species.ndim          
iris.species.isidentical   iris.species.nunique       
```

- [ ] resources
- [ ] isidentical ?
- [ ] iris.db ? No idea what this is.  it's not showing up in `dir(iris)`