- added native INTERVAL type to the dialect. This supports

- added native INTERVAL type to the dialect.  This supports
  only the DAY TO SECOND interval type so far due to lack
  of support in cx_oracle for YEAR TO MONTH. [ticket:1467]
- The Interval type includes a "native" flag which controls
  if native INTERVAL types (postgresql + oracle) are selected
  if available, or not.  "day_precision" and "second_precision"
  arguments are also added which propagate as appropriately
  to these native types. Related to [ticket:1467].
- DefaultDialect.type_descriptor moves back to being per-dialect.
  TypeEngine/TypeDecorator key type impls to the dialect class
  + server_version_info so that the colspecs dict can be modified
  per-dialect based on server version.
- Fixed TypeDecorator's incorrect usage of _impl_dict