[feature] add file size to file listing

Show size of files in listing
Added humanReadable and si options to customize the file size (see `ls`)
Fixed jshint warnings

On branch feature-file-listing
 Changes to be committed:
	modified:   lib/ecstatic.js
	modified:   lib/ecstatic/opts.js
	modified:   lib/ecstatic/showdir.js
Human readable file sizes in listings
Opts for human-readable and si
Document humanReadable options