SparseSeries should have a density attribute

[benjello]

ts = Series(randn(10))
ts[2:-2] = nan
sts = ts.to_sparse()
print sts
print sts.density

AttributeError: 'SparseSeries' object has no attribute 'density'

[changhiskhan]

closed via 95500f0

[benjello]

ts = Series(randn(10))
ts[2:-2] = nan
sts = ts.to_sparse()
print sts
print sts.density

yields the following error
return float(len(self.sp_index)) / len(self.index)
TypeError: object of type 'pandas._sparse.BlockIndex' has no len()