URL-decode DB user/password to allow special chars

[zyv]

Hello,

We have learned the hard way that unfortunately this buildpack doesn't support special characters (specifically @) in usernames and passwords. These characters should be URL-encoded - see e.g. how Django package that adds support for DATABASE_URL handles it:

https://github.com/jacobian/dj-database-url/blob/master/dj_database_url.py#L124

However, the buildpack doesn't decode them and therefore uses wrong passwords for user list and database.

This commit adds decoding to the buildpack and thus fixes the issue /cc @marns93 .

All the best, Yury