support driver specification via entry points

This would include both existing databases with just the driver coming from an entry point, as well as the database+driver coming from an entrypoint.

patch, untested:

diff -r 13897cb81bfcbf24c2e96cd22f61a7d42cff4ca8 lib/sqlalchemy/engine/url.py
--- a/lib/sqlalchemy/engine/url.py  Fri Sep 23 22:25:31 2011 -0400
+++ b/lib/sqlalchemy/engine/url.py  Fri Sep 23 22:33:28 2011 -0400
@@ -104,7 +104,14 @@

             module = __import__('sqlalchemy.dialects.%s' % (dialect, )).dialects
             module = getattr(module, dialect)
-            module = getattr(module, driver)
+            if hasattr(module, driver):
+                module = getattr(module, driver)
+            else:
+                module = self._load_entry_point()
+                if module is None:
+                    raise exc.ArgumentError(
+                        "Could not determine dialect for '%s'." % 
+                        self.drivername)

             return module.dialect
         except ImportError:
@@ -128,7 +135,7 @@
             return None

         for res in pkg_resources.iter_entry_points('sqlalchemy.dialects'):
-            if res.name == self.drivername:
+            if res.name == self.drivername.replace("+", "."):
                 return res.load()
         else:
             return None

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