Rework man generator with new go install format

With previous version, `go install` automatically picks up
package `man` and installs `gen.go` as `main` to the $GOPATH/bin which
is not what is expected. Move man page generator to separate
private folder.

.gitignore

@@ -30,7 +30,6 @@ coverage*.out
 xc-out/
 root/
 
-gen
 man/aptly.1.html
 man/aptly.1.ronn