CDB-3094 some helper methods in user model related to organizations
CDB-3094 assigning database and schemas based on whether the user belongs to an organization or not
Consider the following lines as a way to create a couple of users and assigning them to an existing organization:

```ruby
org = Organization.first

foo = User.new
foo.email = 'foo@cartodb.com'
foo.password = 'testing'
foo.password_confirmation = 'testing'
foo.username = 'foo'
foo.database_schema = 'foo'
foo.organization = org
foo.organization_owner = true

foo.save

org = Organization.first

bar = User.new
bar.email = 'bar@cartodb.com'
bar.password = 'testing'
bar.password_confirmation = 'testing'
bar.username = 'bar'
bar.database_schema = 'bar'
bar.organization = org

bar.save
```

Right now:
foo user cannot import tables from common data but can create tables from scratch that are properly cartodified
bar user cannot import and tables created from scratch are not cartodified
CDB-3094 Enable sharing tables using cartodb.CDB_Organization functions

Consider the following example to share the first table from user foo with user bar

```ruby
foo = User.find(:username=>'foo')
bar = User.find(:username=>'bar')
foo.tables.first.add_read_permission(bar)
```
CDB-3094 Adds a new parameter to importer to be able to specify the destination schema for the imported table
CDB-3094 Points to newest cartodb-postgresql extension
Merge branch 'CDB-3094' into CDB-2891