Replaced the regex-based transformation of CSS to XPath

The library now uses the Symfony CssSelector component, which supports
more cases than the custom logic used previously.
Closes #34
Skip `!important` attributes

Skips the properties when they are marked as `!important`, unless the new attribute is also `!important`. Fixes #57
Don't ignore lock-file'
Merge branch 'stof-use_symfony_css_selector'
Bumped version
Merge branch 'patch-3' of github.com:barryvdh/CssToInlineStyles into barryvdh-patch-3
Updated changelog
Merge branch 'barryvdh-patch-3'
Bumped version
Merge branch 'master' into specificity

Conflicts:
	CssToInlineStyles.php