Rewrite README

Closes #364
Closes #368
Closes #371
[ci skip]
Update gemspec to mirror README

[ci skip]
Unfreeze and protect CollectionDecorator#source

Makes CollectionDecorators behave more like dup'd arrays - you can
modify them independently of the source.
Delegate CollectionDecorator#== to decorated collection

Decorator#== compares sources, so CollectionDecorator#== can just
use a standard array comparison with the decorated collection.

This covers the case that the decorated and source collections
differ because of changes made after decoration.
Merge pull request #406 from haines/collection_decorator

Unfreeze collection decorator
Add example of delegation to source

[ci skip]
Merge pull request #399 from haines/readme

Update readme
Update README.md
update CHANGELOG
bump to beta6
Show installation procedure for beta