Sample space is no longer the Cartesian product of alphabets.
Also, initialization takes outcomes first then pmf now.
Missed a few more unit tests and sample_space modifications.
Merge pull request #28 from chebee7i/samplespace

Sample spaces and rearrange order for initialization.
Add a test for sorting sigma-algebras.
Convert len_cmp -> lexico_key.
Merge pull request #29 from chebee7i/lexico_key

Use key instead of cmp for sorting.