So that any changes made to `.heroku/` within pre_compile (such as
installing additional libraries required for the later pip install) are
not clobbered by the cache being copied over afterwards.

Fixes #320.

* update python.gunicorn.sh to WEB_CONCURRENCY.sh

* don't remove WEB_CONCURRENCY for prefixed 0s

* split gunicorn functionality into its own file

* cleanup comments

* fix var ordering

* add retry to curl

* use proper url for buildpack stdlib

* basics of metrics

* pin to stdlib v2

* measure python installation size

* lots of additional metrics

* add old output methods

* $

* empty commit

* don't unset DYNO (we're running on Heroku)

Fixes #280

* blacklist STACK env var for subshells