Add __enter__, __exit__ methods

[gglanzani]

The methods allow us to use connections and cursors with withs statements.

[gglanzani]

Pinging @jingw directly, hoping for a faster merge :)

[petobens]

@gglanzani can this be added also to the presto part? This client seems to have it implemented: https://github.com/prestodb/presto-python-client/blob/master/prestodb/dbapi.py#L121

I've added #218 in a similar spirit to this PR. Seems to work (at lest with connection object) but it needs review.

[gglanzani]

@petobens sure, it should just work, but I don't have a presto cluster to test it :)

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