Add getter methods to AbstractConnectionCell
Add ConnectionCellTest.java
Add RelayCellTest.java
Add public accessor methods to relay cells

 - Add getAddress() and getPort() to RelayBeginCell

 - Add getData() to RelayDataCell

 - Add getAddress(), getPort(), and getNodeId() to RelayExtendCell
Add relay cell tests for unique relay cells

 - RelayBeginCellTest.java

 - RelayDataCellTest.java

 - RelayExtendCellTest.java