Updates installation instructions

* Snapshot schema is no longer needed
* Osmosis is built via gradle (not ant anymore)
* The osc diff file import was missing the `gz` suffix
* `tiler/owl_tiler.rb` is deprecated. Use `scripts/tiler.rb` instead

This is not sufficient to run OWL yet, but we'll get there.
Merge pull request #21 from thiagoarrais/update-instructions

Updates installation instructions