Add getMinOccurs and getMaxOccurs in SchemaManager

* Included unit tests for getMinOccurs and getMaxOccurs
* Added  XMLSchemaException
* Also added checking if URL is null in schemaManager constructor
Add particleMap attribute to SchemaManager

* Edit findElementsMethod to create also particleMap (earlier it
  included only elementDeclMap)
Add getOccurs for particle. Fix elementPathMap build

* Create XMLSchemaException