Adds support for v2 registry login

summary of changes:

registry/auth.go
  - More logging around the login functions
  - split Login() out to handle different code paths for v1 (unchanged logic)
    and v2 (does not currently do account creation)
  - handling for either basic or token based login attempts
registry/authchallenge.go
  - New File
  - credit to Brian Bland <brian.bland@docker.com> (github: BrianBland)
  - handles parsing of WWW-Authenticate response headers
registry/endpoint.go
  - EVEN MOAR LOGGING
  - Many edits throught to make the coad less dense. Sparse code is more
    readable code.
  - slit Ping() out to handle different code paths for v1 (unchanged logic)
    and v2.
  - Updated Endpoint struct type to include an entry for authorization
    challenges discovered during ping of a v2 registry.
  - If registry endpoint version is unknown, v2 code path is first attempted,
    then fallback to v1 upon failure.
registry/service.go
  - STILL MOAR LOGGING
  - simplified the logic around starting the 'auth' job.
registry/session.go
  - updated use of a registry.Endpoint struct field.
registry/token.go
  - New File
  - Handles getting token from the parameters of a token auth challenge.
  - Modified from function written by Brian Bland (see above credit).
registry/types.go
  - Removed 'DefaultAPIVersion' in lieu of 'APIVersionUnknown = 0'`

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

registry/types.go

@@ -55,14 +55,15 @@ func (av APIVersion) String() string {
 	return apiVersions[av]
 }
 
-var DefaultAPIVersion APIVersion = APIVersion1
 var apiVersions = map[APIVersion]string{
 	1: "v1",
 	2: "v2",
 }
 
+// API Version identifiers.
 const (
-	APIVersion1 = iota + 1
+	APIVersionUnknown = iota
+	APIVersion1
 	APIVersion2
 )