Implement the delete method for JDBCConnector

[qxzzxq]

Is your feature request related to a problem? Please describe.
We'd like to be able to delete rows of a table in the database.

Describe the solution you'd like
As the connection information is given in the configuration, we can use java.sql.DriverManager to create the DB connection. Then we can generate and execute the SQL query with the connection.

If possible, check also if we can retrieve the connection directly from Spark. You may need to read a bit of the Spark source code 😃 (cf. org.apache.spark.sql.execution.datasources.jdbc.JdbcUtils)

The input argument "query" should contain only the condition of a complete SQL request. An exception should be thrown if the query is not well formated.

This issue is similar to #81