Add gitattributes

[VincentLanglet]

Issue #, if available:

Description of changes: Hi @stobrien89, the gitattributes file with the export-ignore option allow to not export some files when creating a release. This way people which are installing this release won't have those files, this is for instance useful for the test folder ; when installing a library we don't need your tests.

I saw you have one in another repo (https://github.com/aws/aws-sdk-php/blob/master/.gitattributes) but there is none here.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[VincentLanglet]

Thanks for the contribution @VincentLanglet. This was an oversight on our part. I'll approve once a trailing forward slash is added to tests.

Done, if I understand correctly

[stobrien89]

Thanks again @VincentLanglet!