BUG: replace not converting dtypes
I believe `replace` should do a `convert_objects(copy=False)` after replacement to provide dtype soft-conversion

```
In [1]: df = DataFrame([['foo','bar','bah'],['bar','foo','bah']])

In [2]: df
Out[2]: 
     0    1    2
0  foo  bar  bah
1  bar  foo  bah

In [3]: m = { 'foo' : 1, 'bar' : 2, 'bah' : 3 }

In [5]: df.replace(m)
Out[5]: 
   0  1  2
0  1  3  2
1  2  1  3

In [6]: df.replace(m).dtypes
Out[6]: 
0    object
1    object
2    object
dtype: object

In [8]: df.replace(m).convert_objects().dtypes
Out[8]: 
0    int64
1    int64
2    int64
dtype: object
```
