Add tests on settings

Hi Jörg, 

I've just add a `JDBCRiverTest` class with tests to check all configuration parameters are correctly set. To do this and in order to prevent using getters I've just implement a `toString` method and check that parameters are correctly set in this string (if you prefer we can define getters).

Furthermore this `toString` may be used to print River JDBC configuration.

Otherwise I've made some minor corrections in README.md.

Cheers

Nicolas