Enable inheritance from HappyMapper classes

This is convenient if an XSD contains inheritance relationships. Can
also be used for multiple (top-level) elements that share the same type.
The subclass will inherit all attributes, elements, etc. from the
superclass and can add to them, independent of the superclass.
Merge pull request #27 from Confusion/inheritance-support

Enable inheritance from HappyMapper classes
fixed default namespaces
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?
keeps Boolean out of global namespace
whitespace nits
Merge pull request #35 from haarts/master

Puts top level Boolean class in module HappyMapper
Merge remote-tracking branch 'dam5s/master'