Add vat_rate field to the Task model
Extend Task#summary to also return the VAT
Add Invoice#vat_summary to return a VAT summary grouped by VAT rate
Modify Invoice#total_amount to take different VAT rates into account
Add VAT rate specific summaries to the template and invoice_form view
Make the VAT rate of a task editable via the form, default to global VAT rate

This is the last commit needed to support VAT rates per tasks (closes: #e7b5a7).

Conflicts:
	stoptime.rb
Move config.yaml to config.yaml.example and ignore it to improve deployment
Merge branch 'development'

Conflicts:
	stoptime.rb
Move add customer button to the header
Rework the project/task list in the customer_form view (closes: #9a33e4)