Add decred support

[dmigwi]

All the major issues are fixed apart from a tiny issue resulting from how decred's (Proof of stake) P.O.S. works. Blocks in decred's block chain are validated or invalidated by the consecutive block mined. We therefore implemented a hack that only allows syncing of blocks with at least one confirming block so as to only push blocks which have been validated to the blockbook's db.

The major concern has been referenced here: #213 (comment)

The decred's best block (doesn't have a confirming block thus can't be synced) txs are listed together with other mempool txs in the mempool page. All the data for this block's txs in the mempool is correct apart from from the block height and the block hash field.

Possible ways to fix this could be:

1. Offer a way to clear txs in an invalidated block as described here: Purge Blocks to a specific height #213 (comment)
2. The fields BlockHeight and BlockHash could be added to Tx struct and before executing this code those two fields provided could be used instead if not empty.

[martinboehm]

Hi, when building decred blockbook, I got the following error:

bchain/coins/dcr/decredparser.go:14:2: cannot find package "github.com/decred/dcrd/chaincfg"

Please add your Gopkg.toml/lock to the PR.

[martinboehm]

I am afraid that you took out too much :) Now I get

vendor/github.com/decred/dcrd/dcrec/edwards/ecdsa.go:17:2: cannot find package "github.com/agl/ed25519" in any of:
	/go/src/blockbook/vendor/github.com/agl/ed25519 (vendor tree)
	/opt/go/src/github.com/agl/ed25519 (from $GOROOT)
	/go/src/github.com/agl/ed25519 (from $GOPATH)
vendor/github.com/decred/dcrd/dcrec/edwards/const.go:10:2: cannot find package "github.com/agl/ed25519/edwards25519" in any of:
	/go/src/blockbook/vendor/github.com/agl/ed25519/edwards25519 (vendor tree)
	/opt/go/src/github.com/agl/ed25519/edwards25519 (from $GOROOT)
	/go/src/github.com/agl/ed25519/edwards25519 (from $GOPATH)
vendor/github.com/decred/dcrd/chaincfg/chainhash/hashfuncs.go:9:2: cannot find package "github.com/dchest/blake256" in any of:
	/go/src/blockbook/vendor/github.com/dchest/blake256 (vendor tree)
	/opt/go/src/github.com/dchest/blake256 (from $GOROOT)
	/go/src/github.com/dchest/blake256 (from $GOPATH)

[dmigwi]

@martinboehm Sorry about that please check it again now. Kindly let me know when we can discuss the issue highlighted in this PR's description.