Clarify documentation on numexpr in eval / query
So the `query` function calls `eval` under the hood, which ends up meaning that the function call ends failing with the following when numexpr is not present.  This means that the default engine (numexpr) will lead to error on systems without having extra dependencies present.

```
ImportError: 'numexpr' not found. Cannot use engine='numexpr' 
```

I see four solutions, which I list in increasing order of difficulty:
1.  Make the `numexpr` dependency much clearer in the docstrings.  Explicitly say that an ImportError will occur if you use the default arguments without having numexpr installed.
2.  Make the python engine default
3.  Let the numexpr engine fallback on the python engine in the case of import error
4.  Make numexpr a required dependency

_edit_ It appears that option (3) is preferred here.  
