fix(classloading): use bpmn model api classloader

use BPMN model api classloader for loading the Schema Resource.

This is required in deployment situations where a different classloader
is used for loading the Bpmn Model Api classes and for loading the Xml
Model Api classes. OSGi, Jboss Modules ...
refactor(bpmn): rename namespace constant

rename activiti namespace constant to camunda namespace constant
feat(ServiceTask): add camunda expression attributes

add support for

* camunda:expression
* camunda:delegateExpression

in ServiceTask and ServiceTaskBuilder
fix(test): attribute size validation

validate attribute assumptions based on attribute list size