RLP: remove signed integer support and other fixes

Decode error messages now say "expected input list for foo.MyStruct"
instead of just "expected List".

* Code = '' if gas < len(D) * 5
* Sha3 gas   10 + 10 * len(D), rounding up 32 bytes
* Sha256 gas 50 + 50 * len(D), rounding up 32 bytes
* Ripmed gas 50 + 50 * len(D), rounding up 32 bytes
* Accounts and value transfers no longer reverted

The documentation for reflect.Value.Index states that it will
panic for out-of-bounds indices. Since go 1.4, it actually panics.

There is no agreement on how to encode negative integers
across implementations. cpp-ethereum doesn't support them either.

Merge branch 'feature/p2p-protocol-interface' of https://github.com/fjl/go-ethereum into fjl-feature/p2p-protocol-interface

Submit code coverage to coveralls only on travis build success