This PR adds `config-demo.toml` to `Dockerfile` which uses the db host
`demo-db`. This is more suited for demo setups. For normal docker installations
the db host is changed back to `db` which is a better sane default.
Reworded `INSTALL.md` and `README.md` for more clarity on docker
installation and configuration.
This commit introduces a `blobstore` package and refactors the existing
upload mechanism. Upload is now handled by `providers` and the two
bundled providers are `S3` and `Filesystem`. `app.Blobstore` initialises
the correct provider based on the configuration and handles `Put`,
`Delete` and `Get` operations.
