Direct help output to the correct interface.

Help should go to STDERR if it's in response to bad arguments or an
error. It should go to STDOUT if it's in response to a user requesting
help via `hk help` or `hk help <cmd>`.

Fixes #84.
tell the user the name of their addon when they've added it

Fixes #57.
no idea why that was Fatal instead of Println
add printFatal() to log an error and exit

Also make printError() use log.Println() instead of log.Fatal().

Change all instances of printError() to use printFatal().
better git remote handling

Find a Heroku app remote even if there is no `heroku.remote` gitconfig
and the remote isn't named "heroku"
resolve heroku-postgresql options to URLs