strip import comments

Go 1.4 allows packages to specify a "canonical" import
path with a comment of the form
    package foo // import "bar/foo"
If the package is imported from a path other than
"bar/foo", the go tool will reject it. We circumvent
this check by removing such comments, so that
dependencies can be imported from their rewritten import
paths inside Godeps/_workspace.

See also http://golang.org/s/go14customimport.

Fixes #156
anchor import comment regexp to beginning of line
sort imports before saving rewrite

Fixes #114.