fix go vet issues
simplify for range loop
Rename printUsage() -> PrintUsage()

Also rename printUsageTo() -> PrintUsageTo()
print short usage on usage errors
improve error msg on scale w/ duplicate dyno type

Fixes #133.
Example -> Examples
cleanly handle nil user in psql
update speakeasy again to avoid reading extra input accidentally
another speakeasy update
document ability to use git remote in -a flag
git remote host from gitconfig

Find a Heroku app remote with a custom hostname from a `heroku.host`
gitconfig. The git host gets picked in this order of preference:

* `HEROKU_GIT_HOST`
* `HEROKU_HOST`
* `heroku.host` in gitconfig
* `heroku.com`