Allow using readable index types

Hi,

This pull request adds the option to provide human readable arguments to ```Moped::Indexes``` methods.

This will enable the following code in Mongoid models:
```ruby
class PullRequest
  include Mongoid::Document
  field :useless_level, type: String

  index useless_level: :asc
end
```

Let me know if you like where it's going so I can make the required changes in documentation both here and in mongoid.

Thanks!