Use modern `str.format()` instead of old-style `%` string formatting

**Source:**
http://docs.python.org/2.7/library/stdtypes.html?highlight=.format#str.format

    str.format() is the new standard in Python 3, and should be preferred to the
    `%` formatting described in String Formatting Operations in new code.

    New in version 2.6.
Fixed test-breaking typo

You must `{{brace}}` yourself when using `{` and `}` within a `.format()`-ed string.  :)
Merge pull request #4 from Zearin/feature/str-format

Use modern `str.format()` instead of old-style `%` string formatting