add argument type(like int or float) support

ref: http://docs.python.org/dev/library/argparse.html#type
unfortunately, the name "type" is already used in dargparse to distinguish
"optional" and "positional" arguments, so "arg_type" is used instead for
the sake of backward compatibility.
follow naming conventions and make it more clear, the example and
document are also updated
Merge pull request #1 from xupeng/type-support

add value type(like int or float) support