This is a major breaking change to web.C, the Goji context object. The
Env key has been changed from a map[string]interface{} to a
map[interface{}]interface{} in order to better support package-private
environment values and to make the context value more compatible with
golang.org/x/net/context's Context.

Since strings support equality, most existing uses of web.C should
continue to function. Users who construct Env by hand (i.e., by calling
"make") will need to update their code as instructed by their compiler.
Users who iterate over the environment will need to update their code to
take into account the fact that keys may no longer be strings.

Provide a standard middleware to set c.Env. Don't include it in the
default stack, however, since the RequestID middleware will end up
allocating Env anyways.

Fixes #11