adapted build.gradle file to add a new sourceSet for integration tests
adapted the EthernetFrameTest to be simpler
moved the NSenderTest from src/testjava to src/integration-test/java
removed EthernetHeader as it isn't needed anymore.
modified build.gradle to make repeatable builds as it wasn't possible
with the additional integration-test sourceSet, but applying it before
the other sourceSets solved the problem.
modified NSenderTest to send a raw data unit and an Ethernet frame.
extracted the checksum algorithm in the EhternetFrame class into a
private method for more modularity