Mongoid 3

I've made a lot of progress torwards porting this gem to use Mongoid 3.

Currently not all tests pass.

unit tests fails with:

```
Failures:

  1) CustomFields::Types::HasMany includes a scope named ordered
     Failure/Error: @author.posts.ordered.send(:options)[:sort].should == [[:position_in_author, :asc]]
       expected: [[:position_in_author, :asc]]
            got: {"position_in_author"=>1} (using ==)
       Diff:
       @@ -1,2 +1,2 @@
       -[[:position_in_author, :asc]]
       +"position_in_author" => 1
     ...

Finished in 1.45 seconds
123 examples, 1 failure
```

integration tests fails with:

```
Failures:

  1) CustomFieldsFor with a bunch of existing posts destroys a field
     Failure/Error: post.respond_to?(:main_author).should be_false
       expected: false value
            got: true
     ...

  2) CustomFields::Types::HasMany filtering/ordering posts filters and sorts the list
     Failure/Error: @author.posts.filtered({ :published => true }, %w(title desc)).map(&:title).should == ['Nude', 'Hello world']
       expected: ["Nude", "Hello world"]
            got: ["Hello world", "Nude"] (using ==)
     ...

  3) CustomFields::Types::Select group_by is an non empty array
     Failure/Error: @groups = klass.group_by_select_option(:main_category)
     NoMethodError:
       undefined method `[]' for nil:NilClass
     ...

  4) CustomFields::Types::Select group_by is an array of hashes composed of a name
     Failure/Error: @groups = klass.group_by_select_option(:main_category)
     NoMethodError:
       undefined method `[]' for nil:NilClass
     ...

  5) CustomFields::Types::Select group_by is an array of hashes composed of a list of documents
     Failure/Error: @groups = klass.group_by_select_option(:main_category)
     NoMethodError:
       undefined method `[]' for nil:NilClass
     ...

  6) CustomFields::Types::Select group_by can be accessed from the parent document
     Failure/Error: @groups = klass.group_by_select_option(:main_category)
     NoMethodError:
       undefined method `[]' for nil:NilClass
     ...

Finished in 8.9 seconds
81 examples, 6 failures
```

I bumped the version to 3.0.0.pre because it will be incompatible with 2.x.x

My debugging abilities weren't enough to understand how to fix `NoMethodError` failures.
The farthest I got was noting that `from_db` method is on that cases called with an attribute parameter that is not remotely complete, it probably has something to do with Moped.

I hope that this get merged and eventually locomotive gains compatibility with mongoid 3.