added support for "byte" data type.

"byte" is a supported numeric data type in elasticsearch, but current pyes does not (as of v0.90.1). When "byte" is used in index mapping, it raises an exception:

    /usr/local/lib/python2.7/site-packages/pyes/mappings.pyc in get_field(name, data, default, document_object_field, is_document)
        601     elif _type == "nested":
        602         return NestedObject(name=name, **data)
    --> 603     raise RuntimeError("Invalid type: %s" % _type)
        604
        605

    RuntimeError: Invalid type: byte

This commit added the support for "byte" type.

Cheers.