Fix broken examples and wrong docstrings in documentation, deprecate old aliases CLong/Long.

[Carreau]

Broken examples (all raised when run as written):

• config.rst: the sample Configurable uses Integer but imports Int
  (NameError)
• migration.rst: the observe_compat example read change['value'], a key
  that does not exist in change dicts (KeyError); use change['new']
• examples/myapp.py: the docstring described a 'shortname' tag feature
  that does not exist anywhere in traitlets (the tag was inert
  metadata); describe Application.aliases, the real mechanism, and
  drop the inert tag

Wrong or incomplete docstrings:

• api.rst claimed old_value is None when a dynamic-default trait is set
  before first read; it is actually the trait type's static default
  (or Undefined). Also fix a doubled word.
• trait_types.rst still described Python 2 int/long behavior for
  Integer/Int/Long despite the package requiring Python 3
• import_item's Returns section claimed it returns a module; for
  dotted names it returns the named attribute, which can be any object
• ArgParseConfigLoader.init carried a copy-pasted Returns section
  describing a Config object; init returns nothing
• UseEnum's example dropped a stale Python 3.4/enum34 backport comment

--

I need to gather stats, but I think maybe we should not deprecate Int.