…lMessages class. 'vendor' is now a kwarg for Imbox, but there's also a lookup dict created in vendors/__init__.py for 'magic' determination of the vendor from the hostname.

… If `Imbox.vendor` is not None, look for a custom authentication error string, and raise it if there's a problem logging in. The default error value is raised if there is no custom string.

created `name_authentication_string_dict` from all subclasses of `Messages` in `vendors/__init__.py`, for the lookup mentioned above.

Implement architecture discussed in martinrusev#131, deciding whether to use `Messages` or a subclass of it in `Imbox.messages` based on the `Imbox.vendor` value.  Use the `folder_lookup` dict, present on `Messages` but only filled in on its subclasses, to select the right folder based on a lowercased "standard" name given as the value of `folder`.

created a blank `folder_lookup` on `Messages`.

filled in the first vendor that subclasses `Messages`, `GmailMessages`.  It contains class attributes `authentication_error_message`, `hostname`, `name`, used for purposes described above, as well as `folder_lookup` which is a dict containing aliases, sometimes several, for Gmail's unique folder naming scheme.  These aliases are meant to be "human-friendly" for intuitive `folder` selection in calls to `Imbox.messages`.

fixes martinrusev#131.

…ndor.__name__ to __all__ from the vendors list.

…lMessages class. 'vendor' is now a kwarg for Imbox, but there's also a lookup dict created in vendors/__init__.py for 'magic' determination of the vendor from the hostname.

added `authentication_error_message` as a class attribute to `Imbox`.  If `Imbox.vendor` is not None, look for a custom authentication error string, and raise it if there's a problem logging in.  The default error value is raised if there is no custom string.

created `name_authentication_string_dict` from all subclasses of `Messages` in `vendors/__init__.py`, for the lookup mentioned above.

Implement architecture discussed in martinrusev#131, deciding whether to use `Messages` or a subclass of it in `Imbox.messages` based on the `Imbox.vendor` value.  Use the `folder_lookup` dict, present on `Messages` but only filled in on its subclasses, to select the right folder based on a lowercased "standard" name given as the value of `folder`.

created a blank `folder_lookup` on `Messages`.

filled in the first vendor that subclasses `Messages`, `GmailMessages`.  It contains class attributes `authentication_error_message`, `hostname`, `name`, used for purposes described above, as well as `folder_lookup` which is a dict containing aliases, sometimes several, for Gmail's unique folder naming scheme.  These aliases are meant to be "human-friendly" for intuitive `folder` selection in calls to `Imbox.messages`.

fixes martinrusev#131.

prevent too much bookkeeping in vendors/__init__.py by adding each vendor.__name__ to __all__ from the vendors list.

…endors