More work on transactions/blocks

Added Apache commons-net as maven dependency for NTP support.

Added SAVEPOINT and ROLLBACK TO SAVEPOINT support to DB class.
Added exists() test to DB class.

Add MessageTransactions, with V1/V3 code in one class instead of very similar code
split across two classes. Update DB schema to add version.

More fleshing out of Assets class.

Fleshing out Block class with parse(), generating balance and signature-related methods.

More javadoc. More tests.

pom.xml

@@ -42,5 +42,10 @@
   		<artifactId>log4j-api</artifactId>
   		<version>2.11.0</version>
   	</dependency>
+  	<dependency>
+  		<groupId>commons-net</groupId>
+  		<artifactId>commons-net</artifactId>
+  		<version>3.3</version>
+  	</dependency>
   </dependencies>
 </project>