Enable input of database objects as a tree of YAML files.

 * docs/{dbtoyaml, yamltodb}.rst: Document new --directory option and
   alternative output/input scheme.
 * pyrseas/database.py (Database.map_from_dir): New function to read
   the files in a directory tree and reconstruct the YAML spec.
   (Database.diff_map): Add schema selection code from yamltodb.
 * pyrseas/dbobject/{cast, extension, foreign, language}.py
   ({CastDict, ExtensionDict, ForeignDataWrapperDict,
   LanguageDict}.to_map): Add default_flow_style=False) to yaml.dump
   calls.
 * pyrseas/dbobject/schema.py (Schema.to_map): When outputting to a
   directory, always output the schema YAML file.
 * pyrseas/yamltodb.py (main): Add new option -d/--directory to
   indicate objects should be input from tree of YAML files rather
   than single file or stdout.  Remove schema selection code.