Fix non deterministic incremental inference

[r9y9]

I forgot to clear buffer property.. Add regression tests.

Results:

> nosetests  --nocapture -v tests -a "issue38"                                                                                                                                 
test_deepvoice3.test_incremental_path_multiple_times ... Variable containing:
 0
[torch.FloatTensor of size 1]
 Variable containing:
 0
[torch.FloatTensor of size 1]

Variable containing:
 0
[torch.FloatTensor of size 1]
 Variable containing:
 0
[torch.FloatTensor of size 1]

ok
test_nyanko.test_incremental_path_multiple_times ... Variable containing:
 0
[torch.FloatTensor of size 1]
 Variable containing:
 0
[torch.FloatTensor of size 1]

ok

----------------------------------------------------------------------
Ran 2 tests in 11.017s

OK