Some improvements

Hi there,

I've made a number of changes, the most significant of which is using the file(1) command to determine whether a file is textual or binary, and its encoding. Although I haven't tested it, I suspect this is much more efficient than using `-[stringWithContentsOfURL]` to slurp in an entire file, especially in pathological conditions like a 1GB text file on a slow network.

I've also moved the project to ARC and Modern Objective-C. This does loose you support for 10.6 32-bit, however.

Finally, thumbnail generation now works, and files are given an appropriate badge according to the heuristic described in the commit message.

I haven't contributed to an open source project before, so please let me know if I'm Doing It Wrong.

Cheers,
Nick