Only begin downloading if directly called

chanarch.py (__main__):
  - Check if chanarch.py is being directly executed before trying to
    download threads. This should help testing classes via imports.
Cleanly handle invalid thread URLS

chanarch.py (InvalidURLError):
  - New invalid URL exception handler

  __init__():
    - Save the URL and a description of what was expected, then keep
      going as a subclass of ValueError.

chanarch.py (ChanThread):
  __init__():
    - Add to the docstring that InvalidURLError can be raised if given
      an invalid URL

  set_thread():
    - Add to the docstring that InvalidURLError can be raised if given
      an invalid URL

  parse_url():
    - Add to the docstring that InvalidURLError can be raised if given
      an invalid URL
    - Raise InvalidURLError if the given URL doesn't match the regex

chanarch.py (__main__):
  - Check for InvalidURLError when creating ChanThreads, and exit with
    an error if caught
Updated ChangeLog

ChangeLog.rst:
  - Add changes for v0.1.2
Release Chanarch 0.1.2

- Cleanly handle and show an error if given an invalid 4chan thread URL
- Allow users to import chanarch without trying to download threads