Remove development files/folders for export

Also allows exclusion of these files when pulling through composer.
More info here: https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production

Closes dompdf#1215

.gitattributes

@@ -12,3 +12,10 @@
 *.svg   text
 
 VERSION export-subst
+
+# Ignore all test and documentation with "export-ignore".
+/.gitattributes     export-ignore
+/.gitignore         export-ignore
+/.travis.yml        export-ignore
+/phpunit.xml.dist   export-ignore
+/tests              export-ignore