Added the ability to remove pseudonyms from the database.

briar-api/src/net/sf/briar/api/db/DatabaseComponent.java

...i/src/net/sf/briar/api/db/LocalAuthorExistsException.java

...i/src/net/sf/briar/api/db/NoSuchLocalAuthorException.java

.../src/net/sf/briar/api/db/event/LocalAuthorAddedEvent.java

...rc/net/sf/briar/api/db/event/LocalAuthorRemovedEvent.java

briar-core/src/net/sf/briar/db/Database.java

briar-core/src/net/sf/briar/db/DatabaseComponentImpl.java

briar-core/src/net/sf/briar/db/JdbcDatabase.java

briar-tests/src/net/sf/briar/db/DatabaseComponentTest.java

briar-tests/src/net/sf/briar/db/H2DatabaseTest.java