Screen delete mod

1. I fix it is an error if the scalar is screen_id.
2. You can delete it with the name of the screen is now available

```
zbx.screens.delete(
  :screen_id => 1
)
```

```
zbx.screens.delete(
  :screen_id => [10, 11]
)
```

```
zbx.screens.delete(
  :screen_name => "foo screen"
)
```
```
zbx.screens.delete(
  :screen_name => ["foo screen", "bar screen"]
)
```
I'm sorry, I was send a pull request successive.
The last commit, I will stand

Please merge this commit.
Thank you.