add support for HTTP basic authentication (#79)

This changeset adds a simple request interceptor that performs HTTP basic authentication.

The HTTP spec isn't very clear on the use of character encodings within this header.
The most common interpretation in servers appears to be to expect ISO-8859-1, so I've
used that as a default, as well as allowing the encoding to be specified.

At @adriancole's suggestion, sun.misc.BASE64Encoder is used for the base64 encoding
rather than pulling an implementation into the Util class.  If we ever run into a JRE that
doesn't provide compatibility with that class, we can eliminate that dependency.