base58: add paranoid return value checks
add NetworkIDString() to chainparams

- returns the BIP70 network string
- use that new function in the core and GUI code and remove unused code
  and functions
Merge pull request #4337

88df548 base58: add paranoid return value checks (Jeff Garzik)
Merge pull request #4326

f5ae6c9 add NetworkIDString() to chainparams (Philip Kaufmann)
Remove `using namespace std` from header file

It's considered bad form to import things into the global namespace in a
header. Put it in the cpp files where it is needed instead.