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.
fixup! New: Testuite created