🌈 sessions were re-written, update to 4.0.0-alpha.2, read HISTORY.md

**Sessions were re-written **

- Developers can use more than one 'session database', at the same time,
to store the sessions
- Easy to develop a custom session database (only two functions are
required (Load & Update)), [learn
more](https://github.com/iris-contrib/sessiondb/blob/master/redis/database.go)
- Session databases are located
[here](https://github.com/iris-contrib/sessiondb), contributions are
welcome
- The only frontend deleted 'thing' is the: **config.Sessions.Provider**
- No need to register a database, the sessions works out-of-the-box
- No frontend/API changes except the
`context.Session().Set/Delete/Clear`, they doesn't return errors
anymore, btw they (errors) were always nil :)
- Examples (master branch) were updated.

```sh
$ go get github.com/iris-contrib/sessiondb/$DATABASE
```

```go
db := $DATABASE.New(configurationHere{})
iris.UseSessionDB(db)
```

> Note: Book is not updated yet, examples are up-to-date as always.

sessions/sessions.go

@@ -1,19 +0,0 @@
-package sessions
-
-import "github.com/kataras/iris/config"
-
-// New creates & returns a new Manager and start its GC
-func New(cfg ...config.Sessions) *Manager {
-	c := config.DefaultSessions().Merge(cfg)
-	// If provider is empty then return nil manager, means that the sessions are disabled
-	if c.Provider == "" {
-		return nil
-	}
-	manager, err := newManager(c)
-	if err != nil {
-		panic(err.Error()) // we have to panic here because we will start GC after and if provider is nil then many panics will come
-	}
-	//run the GC here
-	go manager.GC()
-	return manager
-}