Updating README with basic Uint64 documentation

and some grammar fixes and reduction to basic markdown.
Whoops, titles a bit big
Clarification on number types

Just to clarify that these are not actually uint64 functions but mimic uint64 found in other geohash libraries. Of course the numbers returned are just normal javascript floating point numbers.
Small grammatical/spelling things
Cleaned up code formatting a bit :)
Merge pull request #12 from arjunmehta/patch-4

Updating README with basic Uint64 documentation