Added record_timestamps class var to the timestamps plugin

Hey,

I noticed, that mongo_mapper ignores the ActiveRecord::Base.record_timestamps feature. This is a flag, that allows user to disable/enable timestamps auto-updates, in order i.e. to set some legacy data in the updated_at field. I saw someone had needed something similar in some of the old email threads: https://groups.google.com/forum/?fromgroups=#!topic/mongomapper/bzPA5gEUrKw

My patch works similar as ActiveRecord: It sets the class variable record_timestamps and disables updating timestamps when it's false.

Please let me know what do you think about this.

Best regards,
Kamil Bednarz