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 for `{\"u} \"{u}`.
* (Minor) Use itertools.chain() to simplify code.
* (Minor) Update README to mention Python 2.7.
Merge branch 'crappy-characters-ordering-issue' of github.com:drakeguan/python-bibtexparser into crappy-characters-ordering-issue