Adding & updating documentation

I made some improvements to the README's wording, along with minor
changes to Jaccard's documentation. All methods are reasonably documented
now.
Updating specs

Finished pending specs and added "better" input vectors for each method's
specs (e.g. real data for #haversine, binary arrays for #tanimoto).
Updated README and Rakefile

Rakefile contains a (commented-out) RDoc task to generate documentation.

README now has a section on documentation (also showing how to build it
locally) and lots of other fixes.