Allow Elasticsearch tests with HTTP basic auth.

Added test support for HTTP basic auth and factored out ES settings.

To use a non-standard host and port, use `ESINDEX_TEST_HOST`,
`ESINDEX_TEST_PORT` environment variables to override defaults.

To test HTTP basic authentication use `ESINDEX_TEST_HTTP_AUTH`.

Example running tests against port 9201 with basic auth:

    $ ESINDEX_TEST_PORT=9201 ESINDEX_TEST_HTTP_AUTH='admin:admin' nosetests test/_esindex_test.py