Support Python 2.x.

* Fix compatibility of StringIO.
* Provide an utility fucntion to handle unicode issue in Python 2.x.
Add python2.7 to .travis.yml.
Make Python2.7 happy with unittests.

* Use `unicode_literals` to make str in 2.7 as unicode.
* Function `prepare_unicode_to_latex` is created to wrap generation
  of unicode-latex mappings in consideration of unicode.
Merge pull request #6 from drakeguan/support-python2.x

Support Python 2.7.