Crappy characters ordering issue

Fix accent conversion issue.

Problem: `{\\"u} \\"{u}'}` wouldn't convert to `'ü ü'`.

Reason: The conversion starts from `\\"` instead of `{\\"u}`.

Solution:
    * Make unicode_to_*latex as an ordered tuple.
    * Remove duplicate mapping in unicode_to_crappy_latex1.
    * Add a test of `{\\"u} \\"{u}'}`.
    * (Minor) Use `itertools.chain()` to simplify code.
    * (Minor) Update README to mention Python 2.7.