Adding convenient method to call a block inside a transaction

It's an amazing library and is helping me a lot! Thank you guys for all the efforts on this.

During my development I found myself doing a lot of:

```objective-c
[realm beginWriteTransaction];
//code
[realm commitWriteTransaction];
```

So I thought about adding a convenient method to do a block inside a transaction.
What you guys think? It's a really small change and I want to start contributing a little bit more on the project!
But this is my small start.

Thank you guys in advance.