Add gitattributes file

Used by composer to ignore tests and other development files in the
distribution of a release.

Author: jdufresne

.gitattributes

@@ -0,0 +1,8 @@
+/.gitattributes export-ignore
+/.gitignore export-ignore
+/.scrutinizer.yml export-ignore
+/.travis.yml export-ignore
+/examples/ export-ignore
+/phpmd.xml.dist export-ignore
+/phpunit.xml.dist export-ignore
+/tests/ export-ignore