Fix possible notice.

Suggested by @lucien144.
Improve curl error handling.

Suggested by @rokerkony.
Add common problems to readme file.
Add contact email to readme file.
Improve readme.
Add custom_id option to get_subjects.
Add another example.
Change fakturoid.php to lib/Fakturoid.php.
Add composer.json file.
Improve readme file.
Fix autoload definition for composer.
Merge fakturoid/master