WIP: Introduce a context object on RESTStorage to prepare for future changes

Per feedback on #1417, I have added a Context object to the major apiserver interfaces.

This is a prerequisite to staging in support for enabling the namespace of a request to get set on a context and passed down the call-stack.

Other future potential use cases:

1. request id (traceability)
2. oauth token for request (when user is calling an API that delegates to another call)
3. user unique identifier (per accts, so actions can get traced to a user)
4. etc.

If folks are ok with this general pattern, I will add the Godeps changes in an additional commit.

@lavalamp @smarterclayton 
I'm arguing we don't need any of those except value for the immediate follow up need, and since value is already a method it costs us nothing to not depend on one particular interface from another package (vs one of our own).

> On Sep 25, 2014, at 5:47 PM, Daniel Smith <notifications@github.com> wrote:
> 
> In pkg/api/context.go:
> 
> > +You may obtain a copy of the License at
> > +
> > +    http://www.apache.org/licenses/LICENSE-2.0
> > +
> > +Unless required by applicable law or agreed to in writing, software
> > +distributed under the License is distributed on an "AS IS" BASIS,
> > +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
> > +See the License for the specific language governing permissions and
> > +limitations under the License.
> > +*/
> > +
> > +package api
> > +
> > +import (
> > +	"code.google.com/p/go.net/context"
> > +)
> There's more to a context than just that. See: https://godoc.org/code.google.com/p/go.net/context#Context
> 
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