commons-dbcp2 support

Though commons-dbcp2 is still young, it should be the default CP implementation since ScalikeJDBC 2.1 if it works fine.

FYI:

http://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22org.apache.commons%22%20AND%20a%3A%22commons-dbcp2%22