Interactive Table object

This adds an interactive table object as discussed at https://github.com/ContinuumIO/blaze/issues/298

It is just a TableSymbol with attached data.

I then hook up `compute`, `into`, and monkey patch a `__repr__` onto `Expr` that takes the head of the result, pushes into a Pandas dataframe, and then uses their string printing.  The result is an interactive, multi-backend experience.  Please note that this is not done.  In particular managing imports is starting to become unwieldy.  I think we should transpose the directory structure.

But in the mean time enjoy a data-frame-y interactive workflow powered by sqlite.

```Python
In [1]: from blaze.compute.sql import compute

In [2]: from blaze.data.sql import SQL

In [3]: from blaze.expr.table import *

In [4]: from blaze.api.table import Table

In [5]: sql = SQL('sqlite:///btc.db', 'user_edges')

In [6]: t = Table(sql)

In [7]: t
Out[7]: 
   transaction   sender  recipient            date       value
0            1        2          2  20130410142250   24.375000
1            1        2     782477  20130410142250    0.770900
2            2   620423    4571210  20111227114312  614.174951
3            2   620423          3  20111227114312  128.040520
4            3        3     782479  20130410142250   47.140520
5            3        3          4  20130410142250  150.000000
6            4    39337      39337  20120617120202    0.310818
7            4    39337          3  20120617120202   69.100000
8            5  2071196    2070358  20130304143805   61.602352
9            5  2071196          5  20130304143805  100.000000
...

In [8]: t['value'].max()  # SQL results need to be more Scalar aware
Out[8]: [(500000.0,)]

In [9]: By(t, t['sender'], t['value'].max())
   sender         value
0       1  40000.313370
1       2     38.349483
2       3    150.000000
3       4   7723.000000
4       5    119.000000
5       6     35.000000
6       7   8269.503018
7       8     32.700000
8       9     50.000000
9      10      1.810000
...
```
Yeah, imports are becoming a biggish problem.  I have a refactor in mind that I think will clear this up.

Mostly the problem is trying to isolate dependencies.  In this case we should probably just assume that the user has pandas though.