issue #20 #39 #42 fixed.

add KeyExist()

remove sstream in order to reduce object file size.

1. add template method "ReplaceAdd".
2. redefine typdedef.
3. a version with sstream.

1. Get() compatible with int and double.
2. compatible with float and double.
3. add ResetTraversing().

add null support.

CJsonObject based on cJson