Specify columns in SELECT rather than using readonly option

This is an attempt to deal with #453 and may be reverted.
Made note about finders on assocations

Resolves #452
Use finders module in benchmarks
Support friendly finds on associations in Rails 4.0.1

Resolves #452 for real this time
Test against Rails 4-0-stable; test only on Ruby 2.0

Leaving 1.9.3 out is a compromise to stop the Travis build matrix from
growing too large/slow.
Bump version and update changelog
Fix finder example in scoped documentation
Merge pull request #464 from myabc/patch-1

Fix finder example in scoped documentation
Remove outdate documentation
Updated changelog