added copy to field mapping support for appropriate types

@Mpdreamz per our discussion on which types should support this- based on the elasticsearch source, it looks like only the string, boolean, and binary types actually parse the copy_to field.  Have a look at https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/index/mapper/core/TypeParsers.java, parseField(...), which looks for copy_to, is only called from StringFieldMapper, BinaryFieldMapper, and BooleanFieldMapper.

Closes #708