Revert "Switch to copy-and-swap idiom for operator=."

This reverts commit 45cd9490cd261da31cef84a44d2c587be7e26e99.

Ignored ValueInternal* changes, since those did not produce symbols for
Debian build. (They must not have used the INTERNAL stuff.)

  https://github.com/open-source-parsers/jsoncpp/issues/78

Conflicts:
	include/json/value.h
	src/lib_json/json_internalarray.inl
	src/lib_json/json_internalmap.inl
	src/lib_json/json_value.cpp