Simplify and cleanup proxy creation

In the process of cleaning up the suggested changes in #22, I took the liberty of simplifying the proxy construction somewhat, see 9d31328780eb3a1f25b866fb78244353a7e03946 for details.

While doing so, I made some other minor improvements. I could make separate pull requests for them if you prefer, but I've tried to make the individual commits small enough whit reasonable commit messages.

This means we throw out JRuby 1.6.x support, but I guess no one is bothered. Probably needs a major version bump, though.