Updating README.md with a link to the drop-and-use sdk zip for non-composer developers

See https://github.com/paypal/rest-api-sdk-php/issues/18#issuecomment-22305352
Merge branch 'master' into rest-php-src-downloader

Conflicts:
	sample/README.md
Merge pull request #21 from paypal/rest-php-src-downloader

Updating README with link to the drop-and-use sdk zip for non-composer folks