Changed a few things so we can report errors to users properly in the
config loading system.
- We now return from read_mediagoblin_config both a loaded config and
the validation results
- We now have a helper function generate_validation_report that can
generate a proper validation report saying if there are errors in a
way that's useful to users.
- Moved conf->config in the read_mediagoblin_config function, which
looks nicer IMO.
Uses ConfigObj to open the config file. Also does validation via the
config spec, so defaults are provided, strings are interpolated, types
are converted.
