Added eclipse support for exec plugin

Because of Eclipse's weird lifecycle mappings, the M2Eclipse plugin
doesn't know how to handle maven-exec:exec targets by default.

The M2E pluginManagement attribute was specifically designed to handle
situations like this, so the ome-xml and scifio component poms are now
updated to instruct M2E how to run maven-exec:exec.

To reduce overhead, exec will not run on incremental builds, so a full
clean & build is required to re-generate the xsd-fu sources.
Merge pull request #480 from hinerm/eclipse-xsd-fu

Added eclipse support for maven-exec:exec plugin