Ignore all test and documentation with the "export-ignore" attribute

Triming ~5Mb with composer --prefer-dist

.gitattributes

@@ -0,0 +1,11 @@
+# Path-based git attributes
+# https://www.kernel.org/pub/software/scm/git/docs/gitattributes.html
+
+# Ignore all test and documentation with "export-ignore".
+/docs               export-ignore
+/tests              export-ignore
+/lib/Imagine/Test   export-ignore
+/.gitattributes     export-ignore
+/.gitignore         export-ignore
+/.travis.yml        export-ignore
+/phpunit.xml.dist   export-ignore