Fixed gwt:compile, enhancements for standalone apps

• gwt:compile no longer include compile source roots in the classpath (they've been copied through gwt:import-sources and resources:resources anyway)
• gwt:devmode and gwt:codeserver's projects and modules discovery (when no value has been specific) has been relaxed
• gwt:test's devMode has been removed, as its default value will be switched in GWT 2.8 and it'll be deprecated