implement the abstract base edge Relation

This class deals with models as parts of a relation, while being a
light form of Eloquent Models that supports CRUDding attributes and
saving itself to the database, in addition to utilities like
transforming Eloquent Models to Nodes an exposing some of its data as
Collections of Nodes or Models as requested.
implement EdgeIn relationship

Sets the parent and related nodes as end node and start node
respectively.
implement BelongsTo's associate method+tests

This method will associate a related model to the parent model then
initialize and return an EdgeIn relationship with the corresponding
models.
implement the exists and getConnection methods for edge base relation
add timestamps to Edges and test them out

Created relations now have the ability to have timestamps in case the
parent node has timestamps enabled on it.