WL#4371: General getValue/setValue methods for NDB/Python

For Python programmers, use of methods such as setLong, setInt, setString,
getBytes, etc. might be a bit clunky. Work exists on a getValue() method, but
the API needs to be extended so that getValue(), setValue() and equal() all work
appropriately without explicit typing required. Most likely, the same approach
can later be applied to Ruby, Perl, Lua and PHP.