renamed pointer.refobject to pointer.at; added parents, children to pointers;
moved freevars, globalvars, varnames from source to pointers;
added small test of pointers in test_mixins


git-svn-id: svn+ssh://svn.mystic.cacr.caltech.edu/pathos/dill@633 8bfda07e-5b16-0410-ab1d-fd04ec2748df
Merge branch 'master' of https://github.com/uqfoundation/dill