Added "Install" and "Test" build variants

They appear, along with "Build", in the Command Palette. They have also
been given keybindings. The following can now be done without leaving
Sublime:

Run & Report    OS X            Linux / Windows
-----------------------------------------------
go build        cmd+b           ctrl+b
go install      cmd+alt+b       ctrl+alt+b
go test         cmd+shift+b     ctrl+shift+b