Adds a jwt middleware

It is not certain that it is final, as the jwt is just an interface{}

jwt.go

@@ -0,0 +1,76 @@
+package narco
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/codemodus/chain"
+	"golang.org/x/net/context"
+	"ponyo.epfl.ch/gitlab/alexandre.tuleu/narco/jwt"
+)
+
+type JWT struct {
+	signer jwt.Signer
+}
+
+const (
+	jwtKeyGenerator narcoKey = 2
+	jwtKeyOutput    narcoKey = 3
+)
+
+func (j *JWT) Wrap() func(chain.Handler) chain.Handler {
+	return func(other chain.Handler) chain.Handler {
+		return chain.HandlerFunc(func(ctx context.Context, rw http.ResponseWriter, req *http.Request) {
+			tokenGenerator, ok := ctx.Value(jwtKeyGenerator).(TokenCreator)
+
+			if tokenGenerator == nil || ok == false {
+				//we did not register a token we ignore all processing
+				other.ServeHTTPContext(ctx, rw, req)
+				return
+			}
+
+			// check for the header
+			tokenStr, ok := req.Header["Authorization"]
+			if ok == false {
+				// no Authorization header, we are just not
+				// authentified, we process down, no token added
+				other.ServeHTTPContext(ctx, rw, req)
+				return
+			}
+
+			if len(tokenStr) != 1 || strings.HasPrefix(tokenStr[0], "Bearer ") == false {
+				Error(ctx, rw, fmt.Errorf("Invalid Authorization HTTP Header %v", tokenStr), http.StatusForbidden)
+				return
+			}
+			tkData := strings.TrimPrefix(tokenStr[0], "Bearer ")
+
+			//parse the desired token, with signature checking
+			token := tokenGenerator()
+			err := jwt.DecodeJWS([]byte(tkData), token, j.signer)
+			if err != nil {
+				Error(ctx, rw, err, http.StatusForbidden)
+				return
+			}
+
+			// we process down the pipeline, with the token added to
+			// context
+			other.ServeHTTPContext(context.WithValue(ctx, jwtKeyOutput, token), rw, req)
+		})
+	}
+}
+
+func GetJwt(ctx context.Context) interface{} {
+	return ctx.Value(jwtKeyOutput)
+}
+
+//A Token Creator should produce a new pointer to a desired token
+//struct
+type TokenCreator func() interface{}
+
+// RegisterJwtType should be used to register the token rtyope
+// expected in the context. TokenCreator should allocate a new,
+// unparsed token.
+func RegisterJwtType(ctx context.Context, generator TokenCreator) context.Context {
+	return context.WithValue(ctx, jwtKeyGenerator, generator)
+}