Further refinements to dbtoyaml --directory output. Fixes #52.

 * pyrseas/database.py (Database.map_from_dir): Skip over
   database.*.yaml files.  (Database.to_map): First, read database
   'index' file and remove existing files. Last, write new database
   'index' file.
 * pyrseas/dbobject/__init__.py (DbObjectDict.to_map): New method --to
   replace child methods, writes both individual object map files and
   builds file index map.
 * pyrseas/dbobject/cast.py (CastDict.to_map): Removed.
 * pyrseas/dbobject/extension.py (ExtensionDict.to_map): Removed.
 * pyrseas/dbobject/foreign.py (ForeignDataWrapperDict.to_map):
   Removed.
 * pyrseas/dbobject/language.py (LanguageDict.to_map): Removed.
 * pyrseas/dbobject/schema.py (Schema.to_map): Write individual object
   map files and build file index map.
 * pyrseas/testutils.py (DbMigrateTestCase.setUpClass): Create top
   temporary directory if not present.
 * tests/dbobject/test_extern_file.py: New test to verify proper
   handling of a dropped table.