suggest available package versions in interactive mode

**motivation:** I generally know the package I want, but don't remember the version. Opening a new terminal session and running `composer show -a repo/package` is too much of a hassle.

**changes:** `composer init` (and consequently `composer require`) shows available versions of picked package if it is not specified explicitly.

**remarks:** since the version finder is hardcoded in `show` command, I just changed it to public. This is rather messy and it should be refactored prior to merging this patch with master. This pull request should serve for a discussion regarding the WIP.

*Original vs patched version*:
![original version and patched version](https://f.cloud.github.com/assets/192200/1712641/43eb4bde-6179-11e3-9af6-8f587dc87e60.png)