HOME-Directory Bug, String Readability, Date Validation, Unused Variables, PEP-8

Made following changes to the code:
* Fixed issue with home directory on Windows (Windows has no `HOME` environment variable). Used `os.path.expandvar("~")` instead.
* Transformed the usage-string into a more readable form in the source (put it in a triple-quote-string to be able to remove all the explicit newlines and doublequote-escapes).
* Formatted strings for better readability
    * Using single quotes in string which print double quote, so that they doesn't need to be escaped anymore.
    * Use auto string concat over multiple lines in paranthesis (removed the `+ \` where possible e.g. in `logging.warning(...)`-calls)
    * Used format string instead of concatening the strings manually.
* Added check for a VALID date in the filename (e.g. 2013-37-81 is not a valid date). Also the date doesn't need to be in front of the string - it can be anywhere in the filename now (some digicams put a prefix to their images e.g. img1234...).
* Removed unused variables
* Enforced PEP-8 coding standard (just formatting).