Added eclipse support for maven-exec:exec plugin

This change assumes use of the M2Eclipse plugin for maven integration with eclipse:
http://www.eclipse.org/m2e/download/

With the automated xsd-fu invocation through maven-exec:exec, M2E didn't know how to run the exec goal, which prevents Eclipse use.

This commit adds a pluginManagement specification which allows M2E to handle the exec target.

To test:

Pre-merge:
- In Eclipse, try importing Bio-Formats
(import -> Maven -> Existing Maven projects -> bio-formats top-level).
- Eclipse should complain about maven-exec-plugin:1.2.1:exec. If you continue the import, there will be compile errors.

Post-merge:
- Import Bio-Formats in Eclipse as before.
- Eclipse may request downloading a plugin at this point. Allow it to do so. Restart Eclipse after the plugin is installed.
- If the build is still red after restarting and rebuilding, do a Maven -> update project -> Force update of snapshots/releases.
- After updating maven dependencies & rebuilding, everything should be green.

Tested in Eclipse Juno, service release 2 (OSX)