Proper file naming for DataMapper (datamapper -> data_mapper)

The problem with previous naming is that if we call `"datamapper".classify` it
will return `Datamapper` instead of `DataMapper`.

As I'm trying to get rid of coupling Sorcery with ORM adapters, this has become
an issue in my attempts to implement generic adapter setup
Merge pull request #578 from arnvald/data_mapper_file_naming

Proper file naming for DataMapper (datamapper -> data_mapper)