Modified the build.xml to include a new task for building the dataone
service api package, which generates a jar file that is put into lib. This
task depends on maven being installed in maven.home, and depends on the
DataONE service_api being checed out in a sibling directory to the build.
If these conditions are not met, then the build skips the dataone build and
instead just uses the checked in service-api jar file.