Update properties in models to use correct types

[ghost]

ArkModel

1. Change UnconfirmedBalance & Balance to BigInteger instead of string in ArkAccount

ArkBlock

1. Change PreviousBlock to int instead of string
2. Add reward as BigInteger
3. Add Confirmations as int
4. Add TotalForged as BigInteger

ArkPeer

1. Add Errors as int

DelegateVoters

1. Change Balance to BigInteger instead of string

PeerHeader

1. Change PreviousBlock to int instead of string

[ghost]

Keeping PreviousBlock as string (larger than int64) and changing BigInt to long. The BigInt values can never be larger than Int64.MaxValue.