Make it possible to specify a checksum for the `ark` resource

[wk8]

Which allows to avoid re-downloading the chruby archive from Github at every single Chef-client run; which, besides being nice to Github, avoids stalling Chef on nodes running this cookbook when Github is down.

By default though, the checksum attribute is set to nil (and hence not used) to maintain backward compatibility with existing wrapper cookbooks.

Also properly setting the version attribute of the ark resource, mainly for cosmetic reasons.

Updated the README.