Add support for watch-only addresses

This is a port of the exp-0.8.6.2-watchonly litecoin patches (via Eric Lombrozo, Pieter Wuille et al.) and adds watch-only address support to vertcoin. 

Watch-only support further decentralizes the network by allowing users to track addresses directly from their node rather than relying on block explorer services etc. Its also a good first step towards facilitating vertcoin web wallets and payment services -- private keys can be securely separated from public-facing application functionality such as sending payment and confirmation alerts. The watch-only branch of bitcoin is a requirement to run coinpunk, as an example.