Support for packagist

[pensiero]

Thanks to this PR, users will be able to import your library inside their project thanks to composer and packagist.

In case you approve the PR, you should also create a release (here on GitHub) and submit your repo to packagist.org. I already did this with my repo, in order to use it in my projects. In case I can remove it.

Some extra contained in this repo:

• Added custom exceptions
• Code cleaned and updated (thanks to PHP Inspections plugin)
• Simplified the namespace (OpensslCryptor instead of ioncube\phpOpensslCryptor); I referred to standard conventions that are widely used around open source PHP projects on GitHub (camelcase and only one word); moreover i stripped the word php, that is not useful (we are working within a php environment); if you don't like it, and you prefer something else, don't hesitate to tell me

Fixes #9