Fix use of destroy hooks

INSTANCE_HOOKS returned an empty hash, leading to NoMethodError
Merge pull request #1 from FND/hooks_fix

Fix use of destroy hooks
Add a DataMapper::Is::AwesomeSet::VERSION constant
Bump version to 1.0.2
Remove the dependency on jeweler
Make the parent FK field optional

DM changed the default behavior of `belongs_to` to
be required by default.
Don't permanently modify the class_opts variable
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