Package xml validation

Based on discussion ros-infrastructure/rep#77.

All released packages in G/H/I are still parsable.

Some of them will show the "version convention" warning:
* Groovy
  * libcsm
  * libg2o
* Hydro
  * libg2o
  * ompl
* Indigo
  * libg2o

@tfoote @wjwwood Please review.