cleanup
Implemented "skip k" decoding

At each step, the decoder can either
1. translate an upcoming phrase and append it to the running translation.
2. translate an upcoming phrase and stash it if the stash isn't full.
3. take a phrase out of its stash and append it to the running translation.

Ran with --max-skips=20, --beam-size=100

logprob: -5076.058658