Update speakers model

* Add the nickname field
* Remove session type field in the speaker model
* Change the wording of proposal's kind
* Use south to migrate the database
Update the proposal model

* Change the field type from "foreign key" to "choice field"
Update the proposal form

* Fix the missing reference of foreign key
Modified font family & size of <h1>, <h2>
Merge branch 'master' of https://bitbucket.org/pycontw/pycontw
Remove the prefix of url_pattern. Use nginx instead.
Make the i18n work with the nginx prefix