use "tomcat7-maven-plugin" instead of "tomcat-maven-plugin"

in tomcat6, Matrix variable will fail, such as "/data/matrixvars;foo=bar/simple" will resolve to controller "/simple"
in tomcat6, Async Requests Tab will fail, throw a "java.lang.UnsupportedOperationException: No async support in a pre-Servlet 3.0 runtime"
Merge pull request #17 from hechaoyi/tomcat7-plugin

use "tomcat7-maven-plugin" instead of "tomcat-maven-plugin"