Exclude some dirs and files from repository auto-generated ZIP archiv…

…es (#113)

People that install this library via composer don't need the test stuff, they just need the library itself.
This PR removes the development directories and files from the repository auto-generated ZIP archives.
Since these development files/directories will still be available via `git clone`, Travis is happy, developers are happy, and so are end-users (they won't have unneeded stuff in production machines).

.gitattributes

@@ -0,0 +1,4 @@
+/Tests export-ignore
+/documentation export-ignore
+/.* export-ignore
+/phpunit.xml.dist