Added an @Execute annotation to RunMojo

Update RunMojo to include an @Execute annotation. Allows the use of
`mvn spring-boot:run` without having to compile/package first. Now a
command like `mvn clean spring-boot:run` works.

The phase for @Execute annotation is along the lines of what is used
for the `jetty:run` plugin.
Merge pull request #38 from bijukunjummen/mvnpluginphase

# By Biju Kunjummen
* mvnpluginphase:
  Added an @Execute annotation to RunMojo