Fillna with value & other arguments should raise exception
```
In [1]: import pandas
In [2]: import numpy as np
In [3]: s = pandas.Series(range(10),dtype=np.float64)
In [4]: s[3] = np.nan
In [5]: s[6] = np.nan

In [6]: s
Out[6]: 
0     0
1     1
2     2
3   NaN
4     4
5     5
6   NaN
7     7
8     8
9     9

```

backfilling ok

```
In [8]: s.fillna(method='bfill')
Out[8]: 
0    0
1    1
2    2
3    4
4    4
5    5
6    7
7    7
8    8
9    9
```

here the value argument takes precedence over the method argument (which is ignored)
I propose that this should raise an exception("incompatible arguments to fillna, method is ignored?") 
this applies to Series,DataFrame,Panel

```
In [11]: s.fillna(value=0,method='bfill')
Out[11]: 
0    0
1    1
2    2
3    0
4    4
5    5
6    0
7    7
8    8
9    9
```
