Added DHL sending instructions

Added english translation for DHL sending sinstructions, closes #16
Changed Proce decimals, Session managment and contact form

Price shown with two decimals, closes #12
- tabView
- sell page (we pay you)
Unique Session for each user closes #7
- Check if session is set
- if not create new order ID and store immediately in DB to have it
unique
- destroy session on "confirm" status is set
Contact Form, closes #5
- Unique ids (conflict with feedback form, therefore also unique
functions for sending were needed