Fix for static file saving

There is a line where the thumbnailer creates the directory where the static thumbnail file is saved for later use: [ThumbnailResponder.php#L180](https://github.com/bolt/bolt-thumbs/blob/master/src/ThumbnailResponder.php#L180)

According to [PHP manual](http://php.net/manual/en/function.mkdir.php#refsect1-function.mkdir-errors) we need an extra check if the directory exist, otherwise mkdir throws an E_WARNING level error which translated to Excpetion by Bolt and the file will not be saved at the next line because the try catch block.
Merge pull request #2 from ntomka/master

Fix for static file saving
Don't allow empty width/height/filename in request
Readded autoscaling by setting width or height to 0.
Make sure width/height are positive integers or 0, otherwise use original value.
Removed 0-value from test for bad dimensions, extended testcases.
Added tests for autoscaling.
Merge pull request #3 from rarila/enable-autoresize

Enable autoresize again