Fix NetworkValues prefixes for litecoin

Incorrect values are defined for litecoin testnet for the private and public key prefixes. The pay_to_script value is missing for both litecoin mainnet and testnet. This pull request changes the prefixes to conform to the values defined in litecoin.

https://github.com/litecoin-project/litecoin/blob/master-0.8/src/base58.h#L277