* Namespace `ActiveRecordTest`
* Set `BarnTest` `build_chain`, so memoized builders aren't used
* On `CONTRIBUTING.md`, examples need test lib added to `$LOAD_PATH`
Pass self to each builder, so builder knows the current namespace (Barn or Custom).
If Barn or custom namespace defines constant, get the constant.  Otherwise, look for constant in global namespace.
Namespace User model for safety
Revert "If Barn or custom namespace defines constant, get the constant.  Otherwise, look for constant in global namespace."

The class to instantiate can be set by the user.
Revert "Pass self to each builder, so builder knows the current namespace (Barn or Custom)."

For now, this isn't needed.
Move User fixture into Barn::Builders::ActiveRecordTest and set class to be instantiated.
shortcircuit activerecord builder if no class found
put User fixture in global namespace to make constantize work
Merge remote-tracking branch 'origin/master' into namespace-tests

Conflicts:
	test/builder/activerecord_test.rb
Merge pull request #16 from jch/namespace-tests

Namespace Tests