Release 0.5.1

Enhancements since 0.5.0:

• #26 Provide a way to get a fake servlet-request for Java interop (that needs to call .getParameter()).
• #25 Support remote IP address: remote-addr returns the value Ring obtains.
• #24 Support request / response headers: header is two-arity for reading / writing header values.

Bug fixes since 0.5.0:

• Overhaul of error handling / reporting and layout cascading #27 and #28