Decode correct Content-Type

[jessecarl]

Part of #36

Inspect the Content-Type being provided in the request body and use the correct decoder to unmarshal Resources.

Unsupported request body content type should result in a 415 response.

[jessecarl]

This comes down to mapping http requests to the store methods and other functions exposed by the API. We have the following endpoints:

• NewResource() faststatus.Resource at (GET|HEAD) /new
• store.Get(faststatus.ID) (faststatus.Resource, error) at (GET|HEAD) /{{id}}
• store.Save(faststatus.Resource) error at (PUT) /{{id}}

Given these simple mappings, we only really need a decoder for the store.Save method. One remaining question is what order errors present themselves. I think, for the purpose of this service, the correct path is checked before the correct method.