Support dbname and database kwarg in psycopg2.connect()

[IlyaSukhanov]

*Issue #90

*Description of changes:
psycopg2.connect() Supports 3 variants of operation:

1. psycopg2.connect("dbname=test user=postgres password=secret")
2. psycopg2.connect(dbname="test", user="postgres", password="secret")
3. psycopg2.connect()

Previously we only supported 1 and 2. Now support variant 3 and mix of 2 and 3.

This change is not entierly backwards compatible previously the metadata used to
contain keys url and user. These are no longer exposed however the connection
parameters are available as connection_string. If this backward incompatibility
is not desirable then change can be made to extract values as they were before
using conn.dsn. However the URL will not be correct as password is mangled. It's
not clear to me if one option is clearly better than the other.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[IlyaSukhanov]

Looks like the fields do matter. I'll adjust PR to rebuild them of conn.dsn()

[IlyaSukhanov]

Made change backwards compatible.

[Tankanow]

👍 I like this improvement.

[haotianw465]

Thank you for the contribution. This improvement looks good to me. I think it is a miss on unit test where some use cases are not covered https://github.com/aws/aws-xray-sdk-python/blob/master/tests/ext/psycopg2/test_psycopg2.py. Do you mind adding more common use cases in the test to prevent regression?

[IlyaSukhanov]

Turns out underlying problem was slightly different but fix remained the same. Test has been added and bug report has been updated. I've squashed commits of this PR to remove erroneous description of problem and make it clearer for anyone reading comments in future.

[haotianw465]

Thank you for your contribution. I have merged this PR and will prepare a release soon.