jdbc: Support isWrapperFor() and unwrap() methods

[Totktonada]

It is required by the JDBC standard.

[nicktorwald]

It's required to be implemented at least by following interfaces from the java.sql package

Connection
Statement,
PreparedStatement
CallableStatement
DatabaseMetaData
ParameterMetaData
ResultSetMetaData
ResultSet

[nicktorwald]

Connection, Statement, and PreparedStatement have been already done in the scope of #134