Add support for `with` blocks to Connection

[rshaull]

Support with blocks (__enter__ and __exit__ magic methods) in class Connection (connection.py).

• __enter__ should create a Connection
• __exit__ should close that Connection

The pull request should include a test which demonstrates correct behavior.

[madscientist]

There are also a number of examples in the test suite of using the Python contextlib.closing() facility to provide a poor-man's version of this, which can/should be changed to use this new facility once it's available.

It might be helpful to look at the drivers of other SQL databases implementing PEP 249 and see how they decided to implement this capability (PEP 249 doesn't cover this, itself, but other drivers have added it).

[Vats-shivam]

Raised PR #184 addressing this feature and also have updated the test suite as now it supports context closing without contextlib library