Multiple playgrounds support (#215)

* Add Playground struct and basic support for creating it and retrieving
it

* Add missing functions in pwd mock

* Get playground from request domain and validate it exists. If valid set
it on the newly created session.

* Move playground specific configurations to the playground struct and use
it everytime we need that conf.

* Don't allow to specify a duration bigger that the allowed in the
playground

storage/storage.go

@@ -42,4 +42,8 @@ type StorageApi interface {
 	UserFindByProvider(providerName, providerUserId string) (*types.User, error)
 	UserPut(user *types.User) error
 	UserGet(id string) (*types.User, error)
+
+	PlaygroundPut(playground *types.Playground) error
+	PlaygroundGet(id string) (*types.Playground, error)
+	PlaygroundGetAll() ([]*types.Playground, error)
 }