v3.3 - Update Readme, fix cleanup bug, flip button CSS.

-v3.3.
-Update README.md.
  -Remove mention of Docker repo, as it is not maintained.
  -I appreciate all of the community feedback at #46.
  -I will come up with an official Docker solution.
-Fix bug in cleanFiles() function that was causing problems with archive conversions & preventing cleanup.
-Flip button CSS, so buttons don't appear quite so... upside-down... all the time.
-Remove un-needed (redundant) button related CSS from HRConvert2.css.
-Increase default DeleteThreshold in config.php from 30 to 60 minutes.
-Remove un-needed extension being passed to the convertDrawings() function.
-Fix drawings not returning a valid filename.
  -Conversions would complete, but not download.
-Tested ImageMagick support for SVG, could not get it to work.
-Tested Dia support for converting things into PNG, BMP, and JPG. Got PNG to work, so added support.