decred: Add decred.

[JoeGruffins]

part of #986

This pr adds a decred simple payment verification wallet. It does this by taking code at https://github.com/decred/dcrwallet and packaging it with https://github.com/decred/libwallet into a c library that we then use in cake. All private info is kept locally. The wallet will download and verify all block headers and c filters that it uses to find transactions. Thus initial sync can take a bit, depending on bandwidth. After initial sync subsequent syncs and other wallet functions are pretty fast.

There are still some points that need to be resolved that are listed in the body of #986 but this pr is big enough. We hope to get this into master before starting on those.