Release Notes and version number updated for v0.6
Documentation: Verify docs in new modules

Changes:
- Changed desc-stats to grp_desc_stats to move the
    description information into the right place
- Added grp_summary to mainpage.dox.in and formated summary
    documentation
- Removed 'Parallel Latent Dirichlet Allocation' from mainpage.dox.in
    and added usage for computing perplexity in LDA
- Updated gppkg version to 1.3