Build targets from shards.yml

[tribela]

Sometimes multiple target is needed. bin/compile should build all of it's targets.

[bcardiff]

Yes there might be needed, but the problem is that the user needs to edit manually the Procfile to let heroku know which process is the webserver.

Ideally if there is only one target that should be used without manual Procfile. So the last case of the if can be supported in a natural way (and solving #33).

Also with the PR as is the last case of the if is never reached since the shard.yml is always present.