SparseDataFrame.shift does not respond to axis
The shift method in SparseDataFrame does not respond to axis correctly.

```
>>> import pandas as pd
>>> import numpy as np
>>> df = pd.SparseDataFrame(np.random.rand(10,5))
>>> df.shift(1)
          0         1         2         3         4
0       NaN       NaN       NaN       NaN       NaN
1  0.015673  0.456047  0.451249  0.297851  0.178720
2  0.393355  0.594275  0.496614  0.936675  0.311961
3  0.255445  0.141111  0.750324  0.994727  0.965763
4  0.473577  0.644481  0.584495  0.796462  0.689525
5  0.744432  0.918232  0.715706  0.672892  0.598817
6  0.732707  0.110259  0.756422  0.534970  0.449421
7  0.340564  0.907769  0.543032  0.214241  0.564122
8  0.291026  0.616647  0.102853  0.940241  0.693239
9  0.445545  0.760656  0.035569  0.398808  0.050392

[10 rows x 5 columns]
>>> df.shift(1,axis=1)
          0         1         2         3         4
0       NaN       NaN       NaN       NaN       NaN
1  0.015673  0.456047  0.451249  0.297851  0.178720
2  0.393355  0.594275  0.496614  0.936675  0.311961
3  0.255445  0.141111  0.750324  0.994727  0.965763
4  0.473577  0.644481  0.584495  0.796462  0.689525
5  0.744432  0.918232  0.715706  0.672892  0.598817
6  0.732707  0.110259  0.756422  0.534970  0.449421
7  0.340564  0.907769  0.543032  0.214241  0.564122
8  0.291026  0.616647  0.102853  0.940241  0.693239
9  0.445545  0.760656  0.035569  0.398808  0.050392

[10 rows x 5 columns]
>>>
```

Using Pandas version 0.13.1
