Add the ability to handle HTTP responses where the Transfer-Encoding is chunked.

Add support for making HTTP and HTTPS requests through a proxy.

Add support for downloading files using FTP.

Add new options for controlling the way Urdl uses SSL to implement HTTPS, e.g. the ability to specify a certificate authority file or directory, whether peers are verified, etc. The current implementation always verifies the peer using OpenSSL's default certificate chains.

The current implementation of SSL peer verification supports only certificates issued to a single hostname.

Allow the user to register a boost::function<> object using set_option(). This will be called by the implementation to provide progress updates (e.g. resolving, connection, reading, etc).

Change the underlying implementation to use runtime polymorphism (abstract base classes) to invoke the appropriate protocol implementation. The user will be able to register a factory object to provide custom support for additional protocols.