build scripts update

[lightmare]

Notable changes

• setup.py reads environment variables MASON_ROOT and MAPNIK_CONFIG, which can be used to override these paths; defaults remained as before
• setup.py no longer calls bootstrap.sh; it must be explicitly called earlier
• bootstrap.sh no longer runs postgres, createdb etc.; database initialization has been moved to scripts/setup_postgres.sh
• mason added as git submodule; scripts/setup_mason.sh now tries to update submodule first; if that's not possible, falls back to downloading archive as before

TODO

• [ ] amend README Building from Mason section, add command ./bootstrap.sh