Don't stringify the contents of files before passing them to md5 hasher.

[wbprice]

As demonstrated here, passing a stringified image buffer to the hashing function will yield a different result than if we were to pass an image buffer:

// revisioner.js line 157
    file.revHashOriginal = this.Tool.md5(String(file.contents));

// tool.js lines 65-69
var md5 = function (str) {

    return crypto.createHash('md5').update(str, 'utf8').digest('hex');

};

The above causes images to be hashed inaccurately.

Hash#update accepts a buffer or string as the first argument, when md5 is used again to hash the consolidated hash, it still works.

// revisioner.js lines 310-311
// Consolidate many hashes into one
hash = this.Tool.md5(hash);

Fixes #129