Evolve the api.Status object with Reason/Details

Contains breaking API change on api.Status#Details (type change)

Turn Details from string -> map[string]interface{} - a general
bucket for schemaless error behavior.  Define an open enumeration
ReasonType exposed as Reason on the status object to provide
machine readable subcategorization beyond HTTP Status Code. Define
a human readable field Message which is common convention (previously
this was joined into Details).

Precedence order: HTTP Status Code, Reason, Details. apiserver would
impose restraints on the ReasonTypes defined by the main apiobject,
and ensure their use is consistent.

There are four long term scenarios this change supports:

1. Allow a client access to a machine readable field that can be
   easily switched on for improving or translating the generic
   server Message.

2. Return a 404 when a composite operation on multiple resources
   fails with enough data so that a client can distinguish which
   item does not exist.  E.g. resource Parent and resource Child,
   POST /parents/1/children to create a new Child, but /parents/1
   is deleted.  POST returns 404, ReasonTypeNotFound, and
   Details["id"] = "1", Details["kind"] = "Parent"

3. Allow a client to receive validation data that is keyed by
   attribute for building user facing UIs around field submission.
   Validation is usually expressed as map[string][]string, but
   that type is less appropriate for many other uses.

4. Allow specific API errors to return more granular failure status
   for specific operations.  An example might be a minion proxy,
   where the operation that failed may be both proxying OR the
   minion itself.  In this case a reason may be defined "proxy_failed"
   corresponding to 502, where the Details field may contain a
   nested error object Details["proxy_reason"] that itself is a
   serialization of an api.Status of a different API version.
Nit: perhaps phrase these godoc-style: "ReasonTypeUnknown means that..."