Linux compatability and rounding numbers of cols and rows up instead of down

I had a couple of problems using this tool on Linux (Debian):

1. Case sensitivity caused error when requiring the rmagick lib
2. Dir lib did not work for creating directory structure - switched to Fileutils.mkdir_p which I think is more straightforward anyway.

I did not test these on a platform other than my own.

Also, fixed typing issue when passing tile-height and tile-width param.

Finally,  I was having an issue with some parts of my images not being included in the tiles.  And that fix is included in the second commit.

Thanks for making this tool - it was very useful to me, and I believe I will be using it very much in the future.