update quickstart for Python SDK 0.6.0 change
added Vagrant instruction
fixed compile error msg
refactored dir structure
added gists extension
added concepts:basic
added concepts:data
Cast uid to string before passing to client.identify
Merge pull request #2 from victorhooi/develop

Cast uid to string before passing to client.identify
added concepts:modeling
added concepts:optimization
fixed system design
added diagram to concepts:systemdesign
completed itemrec engine doc
added ItemSim engine doc
added User APIs doc
added Item APIs doc
added u2i APIs doc
added evaluation:baseline
added upgrade tips