Refactor encodings into their own folder.

[drolando]

This will better split out the logic and make it easier to add decoders
in #100

The logic is the same, I just split _encoding_helpers in 2 different files and put them under encoding/
I also had to move some definitions to encoding/_types.py to avoid circular imports.

Existing tests still pass and coverage is still at 100%

[coveralls]

Coverage remained the same at 100.0% when pulling 1624181 on drolando:refactor_encoding_in_its_own_folder into 9090269 on Yelp:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1624181 on drolando:refactor_encoding_in_its_own_folder into 9090269 on Yelp:master.