Use a sed() function for unbuffered output.

So seamless.

Additionally use `cp -a` consistently rather than alternating between
`cp -a` and `cp -r`, and don't fail compile if the glob doesn't expand to
anything.

In `bin/steps/collectstatic` the unbuffered output in `indent` is subverted
by calling `sed` first:

```shell
python $MANAGE_FILE collectstatic --noinput  2>&1 | sed '/^Copying/d;/^$/d;/^ /d' | indent
```

This commit fixes this by making `sed` itself unbuffered rather than
putting that logic in the `indent` function.

fix url to testing buildpack

#117

Fixed Python runtime according to the official Heroku doc

Update Heroku Postgresql default add-on to hobby-dev

See https://devcenter.heroku.com/articles/python-runtimes#supported-runtimes

update shebang to use python2 for bpwatch

Make vendored libraries available at compile time (fixes #57)