Issue #37 - Non-default Keystore Types

jgritman/httpbuilder#37

Modified the AuthConfig class so the certificate() method is overloaded with three different options:
- `certificate(String path, String pass)` behaves as the original method, using the system default keystore type
- `certificate(String path, String pass, String type)` reads the specified file as a user-provided keystore type
- `certificate(KeyStore store, String pass)` uses the user-provided KeyStore; this method allows loading of keys from external devices like SmartCards where a URL may not be available

Also, added support for loading Twitter API configuration from `etc/config/twitter.properties` and supplied a template file detailing the various options. It wasn't clear without delving into the POM that tests were failing because I hadn't configured the Twitter settings. This externalizes them so developers don't accidentally commit auth tokens to the repo.