Add translations to models, admin and forms

[FedeG]

Changes

• Update documentation (Readme.md):
  • Add internationalization documentation to update translation
  • Add internationalization documentation to create new translation language
• Add i18n internationalization to server strings
  • Add verbose_name to all model fields
  • Add Meta class to models with single and plural name
  • Add labels to forms fields
  • Translate choices value on form
• Add string representations to all models
• Add translations to admin actions

Details

@skorokithakis it's the third part of #142

[safwanrahman]

LGTM. r+