BUG: na_values with a dict of scalars
I can't get `na_values` to work with a dictionary.

```
from StringIO import StringIO
import pandas as pd

dta = pd.read_csv(StringIO("""var1,var2,var3,var4
1,2,3,MISSING
2,3,4,4.5
"""), na_values={3 : 'MISSING'})

dta = pd.read_csv(StringIO("""var1,var2,var3,var4
1,2,3,MISSING
2,3,4,4.5
"""), na_values={'var4' : 'MISSING'})

dta = pd.read_csv(StringIO("""var1,var2,var3,var4
1,2,3,MISSING
2,3,4,4.5
"""), na_values=['MISSING'])
```

On

```
[~/]
[10]: pd.version.version
[10]: '0.13.1-753-g4614ac8'
```
