Restructure context types and add AggregateOffer

[madman-81]

1. Create new auto inherit structure for context attributes ($structure). Attributes in the $structure property are now automatically merged with all parent classes
2. The setProperty() function has been renamed to makeProperty() and the actual setting of properties is now down inside the fill() function. This makes for easier unit testing in the future.
3. The makeProperty() is also able to handle nested contexts better:

• If the $value contains @type, it will create a contact based on that type
• $property can now contain an array of classes defining what types of nexted context may be used for that attribute of the main context
• If $value hold and array of contexts, it will automatically make contexts for those. This was done in several context with a setter, these are now all obsolete and therefore removed

4. The Music related contexts have be reorganised and the mutual attributes have been moved to MusicAbstractContext
5. Restructure existing contexts and make the match the hierarchy of schema.org
6. Add PHPUnit config for code coverage
7. Add Json extension to the list of required packages
8. Add AggregateOffer

Solving #70, #68, #63, #49 and #27
And probably also #62,

[madman-81]

This also covers pull requests #67 and #64 making those obsolete