Related to LBCORE-15

- minor refactoring in DBAppender and related classes
- As suggested by John Gibson in LBCORE-15, we now use the prepareStatement method of 
  Connection which takes the sql statement followed by the autogenerateed key columnNames 
  (of type String[])

  This change is compatible with JDBC drivers of SQLServer and MySQL, as well as Oracle.

  JDBC drivers not supporting getGeneratedKeys are not affected by this change.

- added integration tests which are active only on my machine, namely, Orion. This avoids other
  developers from having to install Oracle, MySQL, Postgres and SQL Server just for the 
  logback tests to pass.
  
- similar tests on logback-access to follow.