added license to README.md

Signed-off-by: Markus Andree <me@hapm.de>
added test for correct main class in plugin.yml
fixed wrong main class in plugin.yml
added hamcrest to dependencies to get the additional junit matchers
added VotaPlugin.getDatabaseClasses (refs #14 @5m)

Signed-off-by: Markus Andree <me@hapm.de>