rename matchIncoming() to matchIn() in Eloquent builder
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.
the base edge Relation is now responsible for initialization

initRelation() is no longer an abstract method, it is now implemented
by the base edge Relation that will setup the relationship based on the
$direction attribute which presence is mandatory for every inheriting
edge relation.

Also in this commit, adding passed attributes to the relationship on
init and getting rid of unused $start and $end class properties
test storing different data types preserves types
add the ability to retrieve belongsTo edge

With this commit, we are now able to retrieve relationship edges such
as:
$account->user()->edge(); and we will get the stored edgeIn
relationship, this also implements unique relations where saving new
ones deletes any existing relationship.
update readme with edges and associations instructions
give neo4j some time to start on Travis CI