Pick up system Go even if GOROOT isn't set

Fixes #74
Merge pull request #75 from michaelmaltese/74-read-goroot-from-goenv

Pick up system Go even if GOROOT isn't set
MVP of binary install
Try to detect OS and ARCH. Also cache downloaded binary.
Merge pull request #76 from moovweb/binary_install

Binary install
Bump version
Fix typo