Added build-helper-maven-plugin

Previously, using m2e to import webprotege into Eclipse as an existing
Maven project resulted in compiler errors like
"BioportalProposalsServiceAsync cannot be resolved to a type".  The
build-helper-maven-plugin adds generated-sources/gwt as a source folder
to the project's Java build path, which eliminates compiler errors.
Merge branch 'master' of https://github.com/protegeproject/webprotege