Don't require Railstie unless using Rails 3+

To allow using the gem if your app is tied to a version of Rails older
than 3, this commit makes the require of the Railstie conditional on the
version string.

Initially I had a the conditional require using a regex on the version string,
but then switched to doing a numerical comparison.

Added paragraph into README regarding Rails 2 compatibility.

Also cheekily bumped version to 1.0.4.