Multiline Eval broken for local variables after first line
#### Problem description

As discussed [here](http://stackoverflow.com/questions/42079201/using-local-variables-with-multiple-assignments-with-pandas-eval-function), using local variables with a multi-line eval does not work for locals not on the first line. 

This: 

```python
df.eval("""c = a * @x
           d = b * @y""", inplace = True)
```

fails with:

    error: pandas.computation.ops.UndefinedVariableError: local variable 'y' is not defined

NOTE: I will be submitting a PR shortly.

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 32
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 69 Stepping 1, GenuineIntel
byteorder: little

pandas: 0.19.2
nose: 1.3.7
pip: 8.1.2
setuptools: 18.2
Cython: None
numpy: 1.12.0
dateutil: 2.6.0
pytz: 2016.6.1
</details>
