Fix: Dropbox changed login form action URL

Dropbox has changed the login URL, leading to a 404 for the old one.

Fixes jakajancar/DropboxUploader#37
Version bump v1.1.18

- Fix Login unsuccessful. See Issue jakajancar/DropboxUploader#37
Added composer.json
New: Testuite created

Also adds a test for issue jakajancar/DropboxUploader#18 which reveals
that it does work (wrong report). Runs clean now since more than 15 months.

Also adds a test to make login() testable and test it.