[ENH] SparsePCA: added inverse_transform()

In sklearn.decomposition, TruncatedSVD, PCA, RandomizedPCA, and KernelPCA all have an inverse_transform() method, but not SparsePCA.
@banilo what's this? decimal=0?

a test like this would pass:  ``` assert_array_almost_equal([1, 2], [1,3], decimal=0)```