Add API wiki page, and Markdown-ify it.
Add API wiki page, and Markdown-ify it.

Original: https://hub.joyent.com/wiki/display/dev/NAPI

arekinath/sdc-napi,joyent/sdc-napi,joyent/sdc-napi,arekinath/sdc-napi,joyent/sdc-napi