Caching datasets

A set of changes surrounding computing on datasets

1.  `Data`  is now `InteractiveSymbol`.  `Data` remains a user level function so the public API is unchanged.
2.  `Data('uri::/data/path')` creates `Data('uri').data.path`
3.  Because we now prefer working on database-like-things we also support computing on dicts of data resources
4.  The Server now accepts a single data resource, not a namespace.  However that single data resource can be a dict, which, by point 3, is now a valid computable-thing.  So the public API is unchanged
5.  We add a Cached Dataset thing holds a dataset and caches computations against it in a dict-like object.  This dict-like object can be a [`chest`](http://github.com/mrocklin/chest).

Example
------

```Python
In [1]: from blaze import *

# Setup
In [2]: df = pd.DataFrame([[1, 'Alice',   100],
                         [2, 'Bob',    -200],
                         [3, 'Charlie', 300],
                         [4, 'Denis',   400],
                         [5, 'Edith',  -500]], columns=['id', 'name', 'amount'])
In [3]: iris = resource('/home/mrocklin/workspace/blaze/blaze/examples/data/iris.csv')
In [4]: dset = CachedDataset({'df': df, 'iris': iris}, cache=dict())
In [5]: d = Data(dset)

# Do a little computation
In [6]: d.df.amount.mean()
Out[6]: 20.0

# We keep a cache of expression -> result
In [7]: dset.cache
Out[7]: {mean(_1.df.amount): 20.0}

In [8]: d.iris.species.distinct()
Out[8]: 
           species
0      Iris-setosa
1  Iris-versicolor
2   Iris-virginica

In [9]: dset.cache
Out[9]: 
{distinct(_1.iris.species): (u'Iris-setosa',  u'Iris-versicolor',  u'Iris-virginica'),
 mean(_1.df.amount): 20.0}
```

Of course, future computations of these same expressions are just dictionary lookups.

I haven't yet done any sort of use of intermediate results although, in principle at least, that shouldn't be hard.
what about checking the specific type as well, ie instead of checking what the value isn't, check what it is