Merge pull request #1 from exercism/master

update
editted readme for env install.
editted readme to fix oversized note
typo fix
Update README.md

fixed 2 incorrect steps under setting up environment and development
editted build to differentiate exercism from our build; added download (short: dl) to cmd
Added partial code for the download command and api.go
Add API usage for download; Fix api.go to include code
Finish download feature; tests needed