Add the most common Apache log formats available as class constants

Add the most common Apache log formats available as class constants.
For instance, the user might want to do

```
parser = ApacheLogRegex.new(ApacheLogRegex::COMBINED_LOG_FORMAT)
```

or

```
parser = ApacheLogRegex.new(:combined)
```

via http://www.simonecarletti.com/blog/2009/02/apache-log-regex-a-lightweight-ruby-apache-log-parser/
