Add union support with documentation

Similar to the other basic directives except that unions are stored with
their outside their namespace compound but with their namespace in the
name in the index xml.
Move code out of finder/__init__.py

It seems like it might be a bad idea to have too much stuff in
__init__.py files as it can conflict with modules of the same name
inside the package.
Move code out of finder/doxygen/__init__.py

It seems like it might be a bad idea to have too much stuff in
__init__.py files as it can conflict with modules of the same name
inside the package.
Merge branch 'unions'