Refactored HappyMapper#find

Split HappyMapper#find into three separate methods in the three separate
nodes types. This obviates the need for the #attribute?, #element? and
 #text_node?
Removed the 'to_s' indirection in @attributes and @elements.

It currently doesn't have any function and I'm assuming it
exists for historical reasons.
Merge pull request #25 from Confusion/refactorings

Refactorings
Support for default attribute values

Added a :default option to attribute declarations, which result in a
default value being set on objects. This default value will not be
included in generated XML.
Fixed require in specs