Merge pull request #17 from srikris/how-to-reorg

Organized the how-tos based on categories. Changed the yelp how-to a JSON how-to.
Initial check-in of how-to for Avro ingestion
- added comment to show snippet of SFrame after column renaming
- updated README
Merge pull request #18 from graphlab-code/avro_howto

Initial check-in of how-to for Avro ingestion
Merge branch 'master' into ingress-how-to

Conflicts:
	README.md