Fix many I18n issues and add new features

* Fix many I18n issues
* New features
  - Allow to add creation date as start date
  - Allow to add due date dependent on start date
  - Allow to add description to new tickets
  - Allow to use date-related information
    for subject using variables
You can add a Task like "Maintanance **MONTHNAME** **YEAR**" and it will be added e.g. as "Maintanance March 2014". First line is needed to allow rake redmine:check_periodictasks RAILS_ENV="production" LOCALE="de", so MONTHNAME is substituted with german month name.

This will not break a current installation, except someone uses the defined variables (**WEEK** and so on) as Subject.