updated script events

> added new POST_ROOT_PACKGE_INSTALL and POST_CREATE_PROjECT event, triggered only in create-project
> changed create-package parameter 'package' to be optional, command now looks in working directory for composer project if no param is given

fixes https://github.com/composer/composer/issues/1937

The change with the package param is "debatable" :) Let me know your thoughts!

This is the current output, when running the command in a directory without composer.json file:
```
$ /bin/composer create-project
Composer could not find a composer.json file in /foo/bar
To initialize a project, please create a composer.json file as described in the http://getcomposer.org/ "Getting Started" section
```

PS: And I "had" to use `goto` - feels so C64-style.