protected parent= setter method

Nobody except awesome_set itself should use the :parent= method for manipulating the hierarchy. Now the setter is marked as being protected. This will cause an error if it is called from outside the model.
Fixed missing spaces
Merge pull request #2 from tillsc/protect_parent_setter

Protect parent= setter method
unfreeze version string for Ruby 1.9 compatibility

otherwise `bundle install` fails
Merge pull request #5 from FND/thaw

unfreeze version string for Ruby 1.9 compatibility
Don't create a transaction when there already is one