Issue #15: MERGE support added

[Glitchfix]

https://github.com/RedisLabs/redisgraph-py/issues/15 Feature to merge node or relation added

USAGE:

tim = Node(label='person', alias=client.random_string(), properties={'name': 'Tim Doe', 'age': 25, 'gender': 'male', 'status': 'single'})
redis_graph.merge(tim)

tom = Node(label='person', properties={'name': 'Tom Doe', 'age': 23, 'gender': 'male', 'status': 'single'})
japan = Node(label='country', properties={'name': 'Japan'})
redis_graph.merge("""%s-[v:visited {purpose: "work"}]->%s"""%(tom,japan))

[CLAassistant]

All committers have signed the CLA.

[Glitchfix]

@swilly22 Can you please take a look at it and see if this is okay to merge?

[swilly22]

@Glitchfix Can you please address my question regarding if object:

[swilly22]

@Glitchfix thank you!