update README version to 0.8.0.0

If a developer discovers `react-rails` through Github, they'll probably use whatever version number is listed on the README. For that reason, most Rails projects keep the README version number up-to-date so that folks don't accidentally install old versions.
Merge pull request #20 from matchu/master

update README version to 0.8.0.0
Allow react.js and jsxtransformer.js to be overriden by user
Allow user to override jsxtransformer used for .jsx files
Document manual react and JSXTransformer version changing
Allow for drop-in replacement using recommended files structure
Updated docs with right information on changing react's version
Fixed react.js-replacing paths in comments
Rename test asset files due to collisions
Iron out problems left uncaught during rebase
Bump up version to stop being dependent on react versioning
Updated info about drop-in feature in README