Merge pull request #500 from goutham-M/mifosx-357

 assign staff feature added  for group/center
mifosx-567 ensure duplicate externalId for staff and loan general valid json error response
Merge pull request #501 from johnw65/externalIds

mifosx-567 ensure duplicate externalId for staff and loan generate valid ...
mifosx-567  allow post and put for externalId for loanproduct
Merge pull request #502 from johnw65/externalIds

mifosx-567  allow post and put for externalId for loanproduct
assign staff for group/center(improvement)