Added gxt resources 2.3.1a dependency

http://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22com.extjs%22%20AND%20a%3A%22gxt%22
Added gwt 2.7.0 dependency and gxt 2.3.1a dependency

http://search.maven.org/#search%7Cgav%7C1%7Cg%3A%22com.extjs%22%20AND%20a%3A%22gxt%22