anyconfig should warn if the python module required to parse inputs was missing

[ssato]

Currently, anyconfig.load*() return None and do not warn if it cannot parse inputs because of the lack of python modules to parse them.

see also: issue #42

[ssato]

Just FYI. I'm workin' on this in the warn-missing branch: https://github.com/ssato/python-anyconfig/tree/warn-missing

[ssato]

I decided to suppress logging messages of anyconfig by default now, that is, this behavior won't be changed in the near future because:

• I think the library should not print log message w/o appropriate logging configuration by users
• Setting logging handler other than NullHandler is clearly not recommended: https://docs.python.org/2/howto/logging.html#library-config
• No stable and assured way looks available to suppress warnings about logging handler w/o appropriate logging configuration.
• I prefer to keep the data type of the return value of anyconfig.load*() to maybe object such as Maybe in Haskell/Optional in Java.

I added the note about this behavior and how to configure logging also: 6fa1c2c