Feature/validator email

- Forwarded typo fix `__tostring` => `__toString` 
- Added abstract filter validator, based on [`filter_var`](http://nl1.php.net/manual/en/function.filter-var.php)
- Extended abstract filter validator with `\HylianShield\Validator\Email` using [`FILTER_VALIDATE_EMAIL`](http://nl1.php.net/manual/en/filter.filters.validate.php)
- Bumped version to prepare for release `hylianshield/validator v0.7.0`

@CyberSecutor
```@see http://www.php.net/manual/en/filter.filters.php``` for accepted filter values