Find boxes using go/ast
findBoxes: do not analyze *.rice-box.go files
findBoxes: handle more cases of (Must)?FindBox calls:

 It now detects calls belonging to the rice package, and will ignore
 those that aren't.
 Named imports for rice are supported, as well as ".".
Merge pull request #14 from vincent-petithory/ast-box-finder

Find boxes using go/ast