Gas estimation currently mostly works, but can underestimate for more funky
refunds. This is because various ops (e.g. CALL) need more gas to run than they
actually consume (e.g. 2300 stipend that is refunded if not used). With more
intricate contract interplays, it becomes almost impossible to return a proper
value to the user.

This commit swaps out the simplistic gas estimation to a binary search approach,
honing in on the correct gas use. This does mean that gas estimation needs to
rerun the transaction log(max-price) times to measure whether it fails or not,
but it's a price paid by the transaction issuer, and it should be worth it to
support proper estimates.

The Android NDK was recently removed from gomobile, leading to our Android
builds failing. Starting from https://go-review.googlesource.com/#/c/35173/ ,
gomobile requires a locally installed NDK. This PR ensures that travis installs
that too before running the build steps.

core: removal of dead-code

Removal of dead code that appeard as if we had a consensus issue. This
however is not the case as the proper error catching happens in the vm
package instead.

cmd,eth,les,internal: remove natspec support

* core: Made logging of reorgs more structured, also always log if reorg is > 63 blocks long

* core/blockchain: go fmt

* core/blockchain: Minor fixes to the reorg reporting

accounts, mobile: make account manager API a bit more uniform

common/hexutil: fix EncodeBig, Big.MarshalJSON

The code was too clever and failed to include zeros on a big.Word
boundary.

travis, appveyor, build: add source spell checking

- Use defined constants instead of hard-coding their integer value.
- Allocate secp256k1 structs on the C stack instead of converting []byte
- Remove dead code

tests: unskip test

core: remove support for Olympic network

core: improve import log alignment

vendor: update dependencies

cmd/utils: fix comma-separated --bootnodes

The Azure SDK doesn't support Go 1.5 anymore. We can't upgrade it until
Go 1.8 comes out.

all: fix ineffectual assignments