Remove special handling of "postgis" schema

By default, the PostGIS extension installs PostGIS tables into the
'public' schema. There is no reason for this adapter to do anything
special for a 'postgis' schema. PostGIS tables are excluded from the
structure dump via ignore_tables, not via the schema.

The default postgres #structure_dump method handles multiple schemas
via 'schema_search_path' in database.yml. If you want install PostGIS
tables into a non-default schema, add that schema to your
'schema_search_path' like so:

database.yml:
schema_search_path: 'public, postgis'

https://github.com/rails/docrails/blob/master/activerecord/lib/active_re
cord/tasks/postgresql_database_tasks.rb

http://lists.osgeo.org/pipermail/postgis-users/2009-November/025165.html
Remove schema manipulation

Remove tasks for non-standard schema creation and alteration.

The #setup_gis_grant_privileges and #setup_gis_schemas methods are not
necessary and possibly insecure.
Merge pull request #114 from rgeo/postgis_schema

Remove special handling for 'postgis' schema