Add spend all functionality to wallets

[coveralls]

Pull Request Test Coverage Report for Build 426

• 173 of 253 (68.38%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 54.531%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
bitcoin/wallet.go	0	10	0.0%
litecoin/wallet.go	0	10	0.0%
bitcoin/sign.go	55	69	79.71%
litecoin/sign.go	55	69	79.71%
zcash/wallet.go	0	14	0.0%
zcash/sign.go	63	81	77.78%
<!--	Total:	173	253	68.38%	-->

Totals
Change from base Build 421:	0.7%
Covered Lines:	2780
Relevant Lines:	5098

---

💛 - Coveralls