Allow property.select to override the property field name.

This is a generic way to address
https://github.com/dresende/node-orm2/issues/273
Important: property.select can be a function, whose
return value won't be escaped.
Custom type.datastoreSelect behave like prop.select

This sets a default 'select' for custom types, see:
https://github.com/dresende/node-orm2/pull/375
postgres: fix queries for prop create index, and for primary keys
Merge branch 'pg-index'