Merge pull request #58 from OzzyCzech/master
[TASK] Document how to contribute to Emogrifier.

This change also add a table of contents to the Readme.
Merge pull request #64 from jjriv/task/contributing

[TASK] Document how to contribute to Emogrifier.
[BUGFIX] Restore libxml error handler state after clearing
[TASK] Add Packagist badges to the Readme.
Merge pull request #66 from jjriv/task/badge

[TASK] Add Packagist badges to the Readme.
Add PHP Composer support
Fixed #2