Resolve "Feature: Send endpoint for relations"

- added support for sending relation targets
- internally, relations use forwarding
- templates: add null check before mapping application only if endpoint is not a primitive type
- generation: add default send mapping for java.util.List
- Comiler: always print stacktrace upon compiler error
- build: ragconnect.tests:classes depends on ragconnect.base:jar
- test: checking in RelationTest, should be used in all tests, see #32