Added __enter__ and __exit__ methods to oandapyV20

[ricfaith]

Support with statement from PEP343
Close requests.Session on exit

The unclosed requests.Session was throwing an unclosed socket in my unit tests. Similar findings were found in https://stackoverflow.com/questions/48160728/resourcewarning-unclosed-socket-in-python-3-unit-test
The stackoverflow link shows some other suitable workarounds.

with API(access_token=access_token, environment=env) as client:
    r = trades.TradesList(accountID)
    rv = client.request(r)
    print("RESPONSE:\n{}".format(json.dumps(rv, indent=2)))

[coveralls]

Coverage decreased (-0.3%) to 98.976% when pulling 83a1abc on ricfaith:close-requests-session-with-exit into 3ec71d3 on hootnot:master.

[hootnot]

@ricfaith I'll have a look at it this weekend. Sounds like a good change

[hootnot]

@ricfaith merged your code with a small change: I added a close method allowing to explicitly close the session also.
exiting code can be extended that way by simply adding the line:

client = API(...)
client.request(...)

client.close()

thanks!