Revamp API

• Remove graceful
• Add httprouter
• Tweak consensus, explorer, and gateway routes
• Remove /daemon/update routes
• Overhaul renter routes

httprouter is pretty great, a very minimal and high-performance library that gives us easy access to path parameters like /wallet/transactions/<id>. It also lets us easily enforce proper methods, so I was able to toss out hundreds of lines of method checking code.

All API calls should return structs with lowercase field names now. (No more bare return values.)

I've updated siac to implement these API changes, but Sia-UI (and the explorer) will need some adjustments. We can also discuss whether the changes to various calls seem reasonable.

More details in the commit messages.