TST/API/BUG: resolve scoping issues in pytables query where rhs is a compound selection or scoped variable

e.g. 'l1=selection.index' and 'l1=l', where l = selection.index were failing

example reproduced here: http://stackoverflow.com/questions/20111542/selecting-rows-from-an-hdfstore-given-list-of-indexes

```
In [5]: parms = DataFrame({ 'A' : [1,1,2,2,3] })

In [6]: parms
Out[6]: 
   A
0  1
1  1
2  2
3  2
4  3

In [7]: parms.to_hdf('parms.hdf','df',mode='w',format='table',data_columns=['A'])

In [8]: selection = pd.read_hdf('parms.hdf','df',where='A=[2,3]')

In [9]: selection
Out[9]: 
   A
2  2
3  2
4  3

In [10]: hist = DataFrame(np.random.randn(25,1),columns=['data'],
   ....: index=MultiIndex.from_tuples([ (i,j) for i in range(5) for j in range(5) ],
   ....: names=['l1','l2']))

In [11]: hist
Out[11]: 
           data
l1 l2          
0  0   1.232358
   1  -2.677047
   2  -0.168854
   3   0.538848
   4  -0.678224
1  0   0.092575
   1   1.297578
   2  -1.489906
   3  -1.380054
   4   0.701762
2  0   1.397368
   1   0.198522
   2   1.034036
   3   0.650406
   4   1.823683
3  0   0.045635
   1  -0.213975
   2  -1.221950
   3  -0.145615
   4  -1.187883
4  0  -0.782221
   1  -0.626280
   2  -0.331885
   3  -0.975978
   4   2.006322
```

this works in 0.12
```
In [15]: pd.read_hdf('hist.hdf','df',where=pd.Term('l1','=',selection.index.tolist()))
Out[15]: 
           data
l1 l2          
2  0   1.397368
   1   0.198522
   2   1.034036
   3   0.650406
   4   1.823683
3  0   0.045635
   1  -0.213975
   2  -1.221950
   3  -0.145615
   4  -1.187883
4  0  -0.782221
   1  -0.626280
   2  -0.331885
   3  -0.975978
   4   2.006322
```

This works in 0.13 as well
```
In [16]: pd.read_hdf('hist.hdf','df',where='l1=selection.index')
Out[16]: 
           data
l1 l2          
2  0   1.397368
   1   0.198522
   2   1.034036
   3   0.650406
   4   1.823683
3  0   0.045635
   1  -0.213975
   2  -1.221950
   3  -0.145615
   4  -1.187883
4  0  -0.782221
   1  -0.626280
   2  -0.331885
   3  -0.975978
   4   2.006322
```