Renamed BdfWriter methods, added support for NULL_VALUE.

briar-api/src/org/briarproject/api/data/BdfWriter.java

briar-core/src/org/briarproject/data/BdfWriterImpl.java

...core/src/org/briarproject/forum/ForumPostFactoryImpl.java

...r-core/src/org/briarproject/forum/ForumPostValidator.java

...e/src/org/briarproject/forum/ForumSharingManagerImpl.java

briar-core/src/org/briarproject/invitation/Connector.java

...briarproject/properties/TransportPropertyManagerImpl.java

briar-tests/src/org/briarproject/data/BdfReaderImplTest.java

briar-tests/src/org/briarproject/data/BdfWriterImplTest.java