New: Testuite created

Adds development setup via composer and a phpunit based testsuite.

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.
Fix: logout when done.

Uploading to Dropbox did keep the session open on Dropbox's end.

This has been fixed by logging out on __destruct if the uploader was
logged in.
New: Test for invalid certification file exception
Fix: preserve mtime on upload

The file modifcation time for new and changed files on upload was set to
the start of the UNIX epoch offset by the Dropbox account time zone
setting.

This has been fixed by passing the file mtime as UTC unix timestamp.

This change became necessary due to Dropbox file upload handling around
2014-08-09.