Assign SparseDataFrame row-by-row error
Hi,

I am trying to initialize a sparseDataFrame into block of memory and assign the value row-by-row, which ends up with:

```
a = np.random.rand(4,5)
b = pd.DataFrame(a)
c = sparse.csr_matrix(a)
d = pd.SparseDataFrame(index=b.index, columns=b.columns)
elem = pd.SparseSeries(c[2].toarray().ravel())
d.loc[[2]] = [ elem ]  # Got a NotImplementedError.
elem = pd.Series(c[2].toarray().ravel())
b.loc[[2]] = [ elem ]  # Yes.
```

Seems SparseDataFrame can't be assigned row-by-row, in the manner of assigning DataFrame by DataSeries. Any other method to implement it?
