Exclude non-essential files in .gitattributes (#486)

* Exclude non-essential files in .gitattributes

https://www.reddit.com/r/PHP/comments/2jzp6k/i_dont_need_your_tests_in_my_production/

* Add validation and .gitmodules

.gitattributes

@@ -0,0 +1,19 @@
+* text=auto
+
+/.vscode export-ignore
+/fixtures export-ignore
+/tests export-ignore
+/images export-ignore
+/validation export-ignore
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.gitmodules export-ignore
+/.dockerignore export-ignore
+/.travis.yml export-ignore
+/codecov.yml export-ignore
+/Dockerfile export-ignore
+/Performance.php export-ignore
+/php.ini export-ignore
+/phpcs.xml.dist export-ignore
+/phpunit.xml.dist export-ignore
+/README.md export-ignore