ENH: allow in-line expression assignment with df.eval

This was a relatively easy extension of eval to allow in-line creation/assignment.

Allows one to basically use formulas to do things (pandas conquers excel!!!!)

```
In [11]: df = DataFrame(dict(a = range(5), b = range(5,10)))

In [12]: df
Out[12]: 
   a  b
0  0  5
1  1  6
2  2  7
3  3  8
4  4  9

In [13]: df.eval('c=a+b')

In [14]: df.eval('d=a+b+c')

In [15]: df.eval('a=1')

In [16]: df
Out[16]: 
   a  b   c   d
0  1  5   5  10
1  1  6   7  14
2  1  7   9  18
3  1  8  11  22
4  1  9  13  26
```

You can do this (this could maybe have a bit better syntax though)
```
In [31]: df = DataFrame(dict(a = range(5), b = range(5,10)))

In [32]: formulas = Series(['c=a+b','d=a*b'],index=['a','b'])

In [33]: df.apply(lambda x: df.eval(formulas[x.name]))
Out[33]: 
a    None
b    None
dtype: object

In [34]: df
Out[34]: 
   a  b   c   d
0  0  5   5   0
1  1  6   7   6
2  2  7   9  14
3  3  8  11  24
4  4  9  13  36
```