API/ENH: enable boolean indexing conditions to reference its input frame via eval
http://stackoverflow.com/questions/19938734/is-there-a-simple-way-to-reference-the-current-object-after-applying-a-chain-of

```
df.where("A>0",1)
df.loc["A>0"]
```

so could process the condition via an 'eval' type of operation
