Add 'socket' param to override mysql socket location

See https://github.com/jcabi/jcabi-mysql-maven-plugin/issues/10 and https://github.com/jcabi/jcabi-mysql-maven-plugin/issues/50

Note: I haven't been able to run `mvn clean install -Pqulice` because the tests don't work on my machine (Fedora 20).