ENH/BUG: Sparse now supports comparison op
- [x] no open issue
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
## on current master

```
pd.SparseArray([1, 2, np.nan]) > 0
# [True, True, nan]
# Fill: nan
# IntIndex
# Indices: array([0, 1], dtype=int32)

# Expected
# [True, True, False]
```

```
a = pd.SparseArray([1, 2, 0])
b = pd.SparseArray([0, 1, np.nan])
a > b
# ValueError: operands could not be broadcast together with shapes (3,) (2,) 
```
