Update to FAKE.2.1.97-alpha and enable assembly info task
Refactored model

* remove interfaces from model
* do not inherit from XElement, use now a internal property
* implemented a LicenseAttributes class for all attributes, like product features and customer
* implemented save and load methods for the new license class
fix nunit path
Update readme
Add overloads to configure the license holder with a delegate
Add configuration options for the additional license attributes
Fix typo
Removed Model namespace, its not needed anymore