Handle registered device id in POST /v1/account/device

[philbooth]

curl -v \
-X POST \
-H "Host: api-accounts.dev.lcip.org" \
-H "Content-Type: application/json" \
-H 'Authorization: Hawk id="d4c5b1e3f5791ef83896c27519979b93a45e6d0da34c7509c5632ac35b28b48d", ts="1373391043", nonce="ohQjqb", hash="vBODPWhDhiRWM4tmI9qp+np+3aoqEFzdGuGk0h7bh9w=", mac="LAnpP3P2PXelC6hUoUaHP72nCqY5Iibaa3eeiGBqIIU="' \
https://api-accounts.dev.lcip.org/v1/account/device \
-d '{
  "id": "0f7aa00356e5416e82b3bef7bc409eef",
  "name": "My Old Phone"
}'