SSL server certificate validation

Added code to allow the DNSDB client to:
* Optionally perform browser-like validation of the SSL certificate presented by a server;
* Allow optional provision of explicit list of certificates to accept (typically self-signed certificates).

Also updated the configuration file reader to allow for two new variables `VERIFY` and `CA_BUNDLE` to use the new functionality.

Was able to implement this with very little code by migrating from `urllib2` to `requests` for HTTP queries.