python2.6 compatible format strings

G'day!

Very small patch -- the recent changes to logging in kivy/core/__init__.py are using the new string format() with empty field specifiers, which are supported by Python 2.7 onwards. This patch supplies the field indices, so it will work with Python 2.6.

Really enjoying this project, thankyou for your efforts. The API is so well thought-through, an absolute joy to work with.