Lowercase package paths.

In running through the hellobender tutorial on Mac OSX, I was getting
case-insensitive import collision errors with `go install
$PKG/hellobender`.

Per this thread
https://groups.google.com/forum/#!msg/golang-nuts/q1U6uYfUfy4/HLgGilyODTsJ,
it seems more straightforward to just switch everything to lowercase
paths here.