Add index support

[cedricfung]

The $elemMatch selector won't use multiKey index.

For issue#45, Ruby 1.9 is now more popular than 1.8, so we shouldn't
sacrify performance for it. If we want to support 1.8, consider
OrderedHash from ActiveSupport

[dblock]

Are you saying this will break Ruby 1.8.x? It looks like it. In which case we can maybe do a programmatic if RUBY_VERSION?

[cedricfung]

@dblock Indeed the OrderedHash from ActiveSupport is doing this, we can use OrderedHash if don't mind another dependence.

[dblock]

I don't mind another dependency. Go for it. We can't break existing users though, even if they are stuck in the 20th century Ruby :)

[cedricfung]

So we have three simple choices

1. Create different branches and versions for 1.8.x and 1.9+
2. Use $elemMatch for 1.8.x
3. Use OrderedHash

Maybe someone find better solutions.

[cedricfung]

OK, I will try an OrderedHash commit.

[cedricfung]

Ruby 1.8.x is supported, all specs are green.

I found ActiveSupport is already the dependecy with ActiveSupport::Concern

[dblock]

Ok, thanks, merging.