Added gwt.compiler.skip option.
According to https://github.com/gwt-maven-plugin/gwt-maven-plugin/blob/master/src/main/java/org/codehaus/mojo/gwt/shell/CompileMojo.java#L64 gwt compilation can be skipped with -Dgwt.compiler.skip
Merge pull request #34 from mwl/master

gwt.compiler.skip option