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.

Additionally added a new test for invalid certification file exception.
Fix: preserve file modifcation time on upload

Due to changes in the Dropbox file upload handling around 2014-08-09 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 Unix timestamp.

Additionally a test script for PHP filemtime() has been added for
compatibility checks.

Fixes jakajancar/DropboxUploader#40
Fix: CS

Set docblock file-comment width to 120.