Anonymous mapper

As per issue #13 this provides anonymous XML mapping.

## HappyMapper.parse(XML)

With no classes or configuration you can parse the example XML with little effort:
```ruby
address = HappyMapper.parse(ADDRESS_XML_DATA)
address.street # => Milchstrasse
address.housenumber # => 23
address.postcode # => 26131
address.city # => Oldenburg
address.country.code # => de
address.country.content # => Germany
```

It is important to be aware that this no configuration parsing is limited in capacity:

* All element names are converted to accessor methods with underscorized names
* All value fields are left as String types
* Determining if there is just one or multiple child elements is hard, so it assumes it is one until it finds another with the same name.