Add Daala's Entropy Coder to Thor

[tterribe]

[metajack]

No comment about the results?

[tterribe]

This doesn't convert any of the existing symbols to use arithmetic coding, so the only significant changes in results are due to the extra framing added in the first commit (see that commit for details).

[thdav]

Merged except for the actual addition of the entropy coder. Please can you rebase on the current master again and I will push to a new branch. It might also be an idea to reverse the Daala convention, and write raw bits forward and arithmetic coding bytes backwards.