Add maven-base directory for maven build.

Copy the astrogrid/maven-base directory from the original AG repository
as a subdirectory of this JSAMP repository.  This is enough for the
build; I don't really know what's in there, probably a lot more than
JSAMP needs.
Reference maven-base in its new location from project.xml.

maven-base is now a subdirectory of the JSAMP top level to keep it
tidier, so reference it there rather than having it at ../maven-base.