Added Documentation for GreenPlum

Added a documentation for GreenPlum, including detailed installation guide according to Ce's suggestion. I only finished the installation and configuration for Mac OS X version. If needed, we can add other OS, but they should be similar.

I can successfully run OCR, SMOKE and TITLE examples in an older version of system. With the latest version, I found bugs in compiling, see Issue #38. 

KBP example seems to have some problems with GreenPlum, see Issue #39.