small fix

module attribute definition is now necessary before usage so define @ctx
matching order was wrong in wrap_response function
Ewebmachine is now only a library

Update documentation and mix.exs:
now ewebmachine is only a lib, OTP application do not launch any
supervison tree. You can create your own webmachine supervisor with the
routes of your ewebmachine modules. Or use the default supervisor,
taking ewebmachine modules and connection parameters, as documented at
the end of the README.