* whisper: wnode updated for tests with geth

* whisper: updated processing of incoming messages

* whisper: symmetric encryption updated

* whisper: filter id type changed to enhance security

* whisper: allow filter without topic for asymmetric encryption

* whisper: POW updated

* whisper: logging updated

* whisper: spellchecker update

* whisper: error handling changed

* whisper: JSON field names fixed

This PR implements a differenceIterator, which allows iterating over trie nodes
that exist in one trie but not in another. This is a prerequisite for most GC
strategies, in order to find obsolete nodes.

Removed exported statedb object accessors, reducing the chance for nasty
bugs to creep in. It's also ugly and unnecessary to have these methods.

travis: only run go vet and misspell on latest Go

travis, appveyor: update builders to Go 1.8

* cmd/geth: added 'geth bug' command

Added bug command to geth, which will open a browser window
with an issue template and some additional system information.

* cmd/geth: update bug with better infos

* cmd/geth: added browser fallback

* cmd/geth: govet yo momma

accounts/usbwallet: swap karalabe/gousb to karalabe/hid

* vendor: add github.com/btcsuite/btcd/btcec

* crypto: add btcec fallback for sign/recover without cgo

This commit adds a non-cgo fallback implementation of secp256k1
operations.

* crypto, core/vm: remove wrappers for sha256, ripemd160

swarm/api/http: fix go vet issue on Go 1.8

vendor: update dependencies with github.com/kardianos/govendor

params: core, core/vm, miner: 64bit gas instructions

* whisper: mailserver test introduced, refactoring

* whisper: validation test updated

* whisper: max number of peers fixed

* whisper: verification bug fixed

* whisper: esthetic fix

* whisper: interface changed to simplify the transition to v5

* whisper: preparation for version switch

Reworked the EVM gas instructions to use 64bit integers rather than
arbitrary size big ints. All gas operations, be it additions,
multiplications or divisions, are checked and guarded against 64 bit
integer overflows.

In additon, most of the protocol paramaters in the params package have
been converted to uint64 and are now constants rather than variables.

* common/math: added overflow check ops
* core: vmenv, env renamed to evm
* eth, internal/ethapi, les: unmetered eth_call and cancel methods
* core/vm: implemented big.Int pool for evm instructions
* core/vm: unexported intPool methods & verification methods
* core/vm: added memoryGasCost overflow check and test

* core,eth,internal: Added `debug_getBadBlocks()` method

When bad blocks are discovered, these are stored within geth.
An RPC-endpoint makes them availablewithin the `debug`
namespace. This feature makes it easier to discover network forks.

```

* core, api: go format + docs

* core/blockchain: Documentation, fix minor nitpick

* core: fix failing blockchain test

params: 1.5.9 stable

Dockerfile: support building USB on Alpine, ignore temp files