BUG: fix NDFrame.as_blocks() for sparse containers

SparseBlocks don't consolidate, so previous implementation silently dropped all but the last blocks for given dtype:

```python
In [1]: pd.__version__
Out[1]: '0.13.1-527-g73506cb'

In [2]: pd.SparseDataFrame({'a': [1,2,3, np.nan, np.nan], 'b': [1,2,3, np.nan, np.nan]})
Out[2]: 
    a   b
0   1   1
1   2   2
2   3   3
3 NaN NaN
4 NaN NaN

[5 rows x 2 columns]

In [3]: _2.blocks
Out[3]: 
{'float64':     b
0   1
1   2
2   3
3 NaN
4 NaN

[5 rows x 1 columns]}
```

when the last output should be:

```python
In [3]: _2.blocks
Out[3]: 
{'float64':     a   b
0   1   1
1   2   2
2   3   3
3 NaN NaN
4 NaN NaN

[5 rows x 2 columns]}


```

This also drops the `columns` kwarg of as_blocks since it doubles reindex functionality.