eval should handle ternary operator
It would be nice to be able to write ternary operator into expressions submitted to `eval`. 
#### Code Sample

``` python
df = pd.DataFrame({'a': range(10), 'b': range(10,20)})
df.eval('x = a if a < 5 else b', inplace = True)
df

Traceback (most recent call last):

  File "<ipython-input-27-89e215206825>", line 1, in <module>
    df.eval('x = a if a < 5 else b')

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/core/frame.py", line 2209, in eval
    return _eval(expr, inplace=inplace, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/eval.py", line 245, in eval
    truediv=truediv)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 709, in __init__
    self.terms = self.parse()

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 726, in parse
    return self._visitor.visit(self.expr)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 310, in visit
    return visitor(node, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 316, in visit_Module
    return self.visit(expr, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 310, in visit
    return visitor(node, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 494, in visit_Assign
    return self.visit(node.value, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 310, in visit
    return visitor(node, **kwargs)

  File "/home/benjello/.local/lib/python2.7/site-packages/pandas/computation/expr.py", line 201, in f
    "implemented".format(node_name))

NotImplementedError: 'IfExp' nodes are not implemented
```
#### Expected Output

```
  a   b   x
0  0  10   0
1  1  11   1
2  2  12   2
3  3  13   3
4  4  14   4
5  5  15  15
6  6  16  16
7  7  17  17
8  8  18  18
9  9  19  19
```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.11.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-1-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8

pandas: 0.18.0
nose: 1.3.7
pip: 1.5.6
setuptools: 18.8
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: None
xarray: None
IPython: 4.0.3
sphinx: 1.3.6
patsy: 0.4.1
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.5.2
matplotlib: 1.5.1
openpyxl: 2.3.0
xlrd: 0.9.4
xlwt: 0.7.5
xlsxwriter: 0.7.3
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
httplib2: 0.9.1
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
```
