Improve Config Parser implementation

[instantshare]

Improve Config Parser implementation

[dtext]

After looking at this for a while, there doesn't seem to be very much we can do if we keep using configparser. I'd change a few smaller things:

• Remove logging from config.py:
  This gets rid of the bug where we get an error message when there is no config file because of logging not being initialized when the default config is loaded. Doesn't really make any sense to log that info anyway, I think.

• Handle parsing errors:
  At the moment, instantshare just dies with an exception when there are parsing errors. We should make the config parser more resilient and show the user what's wrong with the file and why we can't continue. ConfigParser itself raises some useful exceptions. We could re-raise them and handle them somewhere else or handle them in config.py. However, the latter is difficult because we need to decide how to give feedback to the user.

• Put everything at module level:
  I don't think it's too bad to have a singleton here, but we should make all the functionality accessible via config instead of config.CONFIG if we're going to keep it this way, in order to hide the implementation details inside the module. This makes using the class a bit nicer.

Alternative
An alternative would be to replace configparser with something more flexible and modern like PyYAML, which can parse YAML strings and convert them to basic python types directly (dict, list, int, str, ...). I have used YAML in a few projects now and I like it a lot.