"*.STL" files might be STL files too

Makes STL file detection based on extension work case insensitive. Without
that patch, files named like "FILE.STL" or "file.StL" or ... couldn't be
sliced. CuraEngine quit attempts to do so with "Could not load file"
instead.
Merge pull request #128 from foosel/fix/fileExtension

"*.STL" files might be STL files too