Union type processing added to JsonMapper

Union type processing added to JsonMapper

if data type according to schema is union then data from single node are validated as follows:
- if simple node value is null -> [null] is written to JSON
- if simple node value is number and union yang contains one of number types ->
  number is written to JSON
- if simple node value si true or false and union yang contains BooleanType ->
  true or false is written to JSON
- other cases are written to JSON as string (inside "")

Change-Id: I6bc2631bc9b018e9497464fac4efbc5abc726882
Signed-off-by: Jozef Gloncak <jgloncak@cisco.com>