seamless - now with multiple backends

Updated seamless to support multiple backends (thank you Fabrizio for helping out).

The new API is:

• /set?backends=host:port,host:port
• /add?backend=host:port
• /remove?backend=host:port
• /get

The front end will round-robin between the backends.

You can install either via go get bitbucket.org/tebeka/seamless or by downloading the binaries.