BUG: SparseSeries.shift may raise NameError or TypeError
- [x] no existing issue
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixed following bugs on current master. Also, moved `TestSparseArrayIndex` to `test_libsparse`
#### 1. shift

```
pd.SparseSeries([1, 2, 3], fill_value=0).shift(1)
# NameError: global name 'kwds' is not defined

pd.SparseSeries([1, 2, 3]).shift(1)
# TypeError: %d format: a number is required, not float
```
#### 2. dtype

```
from pandas.sparse.array import IntIndex
arr = pd.SparseArray([1, 2], sparse_index=IntIndex(4, [1, 2]), dtype=None)
arr.dtype
# dtype('int64')

arr.values
# array([-9223372036854775808,                    1,                    2,
#        -9223372036854775808])

# Expected outputs
arr.dtype
# dtype('float64')

arr.values
# array([ nan,   1.,   2.,  nan])
```

thanks!

sparse is really looking nice!
