support for nested resources

As discussed before I added support for nested resources, including tests and readme.

The parent child relation is now stored using prefix ('R_') to make sure the parent ID's are stored as STINGS i.s.o floats. Floats will not gave enough precision to resolve the parent child relationship on retrieval .... I also prefixed the attribute name designating a parent model ('BELONGS_TO_') to easily differentiate those from other attributes