Rename host to env

I was trying to avoid ambiguity given Rack's heavy use of env but host was
actually more confusing.
Use __getobj__ in nested DSL call

Using self would have caused future self-schizophrenia, so I'm changing it now
that it's on my mind...
Destroy development mode

Treating assets differently in development mode was a failure. It caused many
headaches, filled the codebase with special cases and ultimately felt a little
bit too much like Rails magic. Goodbye.
Kill pointless delegate method

More cleaning allowed by killing development mode :beer:
Kill further pointless delegate methods

:beer: :beer: