Updated README

Edited section on modes in description to reflect as_json instead of to_hash.

Added section regarding Rails compatibility, created new heading for Compatibility.

Broke out the JSON Processing Alternatives section, and edited.
A bit more editing

Read it over a bit more and edited a few more things, changed the formatting a bit.
Hope you don't mind and find this helpful.
more edits top rails section

take -> make, and more specifics for how to get Oj working with which version.
Merge pull request #141 from kookster/master

README edits