Make installable as a global Composer package

[samwilson]

I'd like to suggest that the standard (non-development) installation procedure for Flickr CLI should be:

composer global require --no-dev thefox/flickr-cli

More info: https://getcomposer.org/doc/03-cli.md#global

The main change we'd need to make would be to move application.php to bin/flickr-cli and add it as a vendor binary.

This would not have to be a breaking change, as we could still have application.php, until the next major version bump.

[TheFox]

I would rather also prefer the usage of bin directory. This is a more clear way.