Fix #6 - insert/update of nulls in Clob and Blob fields

API additions to support insert/update of nulls in Clob or Blob fields (to fix #6) :

```java
Database.NULL_CLOB
Database.NULL_BLOB
Database.toSentinelIfNull(String)
Database.toSentinelIfNull(byte[])
```
and new methods on the QueryUpdate builder:
```java
QueryUpdate.parameterClob(String)
QueryUpdate.parameterBlob(byte[])
```

See Clob and Blob sections of the README.md for usage examples.