Feature: Enclosure

This PR implements `initially()` and `ultimately()` as per https://github.com/xp-forge/sequence/issues/8#issuecomment-68205709

```php
Sequence::of($zip->entries())
  ->initially(function($first) use($progress) { $progress->started(); })
  ->ultimately(function($last) use($progress) { $progress->done(); })
  ->each(function($entry) use($progress) { /* Invoked for every element */ })
;
```