Support for PostgreSQL schemas

[olt]

Use configurable schemas instead of the public schema.
https://bitbucket.org/olt/imposm/issue/2/import-data-in-another-schema

[dkastl]

I would like to back this issue. A database is much easier to maintain if it doesn't store everything in the public schema.
Why not use different schemata instead of --table-prefix, --table-prefix-production and --table-prefix-backup?

[gislars]

I'm also in favor of using other schemes. The prefixes could stay, but would be used in the configurable schema.