This commit converts the dependency management from Godeps to the vendor
folder, also switching the tool from godep to trash. Since the upstream tool
lacks a few features proposed via a few PRs, until those PRs are merged in
(if), use github.com/karalabe/trash.

You can update dependencies via trash --update.

All dependencies have been updated to their latest version.

Parts of the build system are reworked to drop old notions of Godeps and
invocation of the go vet command so that it doesn't run against the vendor
folder, as that will just blow up during vetting.

The conversion drops OpenCL (and hence GPU mining support) from ethash and our
codebase. The short reasoning is that there's noone to maintain and having
opencl libs in our deps messes up builds as go install ./... tries to build
them, failing with unsatisfied link errors for the C OpenCL deps.

golang.org/x/net/context is not vendored in. We expect it to be fetched by the
user (i.e. using go get). To keep ci.go builds reproducible the package is
"vendored" in build/_vendor.

This commit tweaks the debian packaging tool:

* All build environment metadata can now be overriden on the command
  line. This allows testing the CI build behaviour locally.
* -unstable packages now actually contain the binaries (oops)
* packages use Go 1.7 to build
* archiving is skipped for PR builds

TravisCI and AppVeyor run the tests in very slow VMs.
Some of our tests can't cope with that. Running less tests
in parallel should make them somewhat less flakey.

This change also deletes generator.go, moving the only interesting line
in it into release.go. The binding has been regenerated with abigen from
develop and solc v0.3.6.

The new build script, ci.go, replaces some of the older shell scripts.
ci.go can compile go-ethereum, run the tests, create release archives
and debian source packages.

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

crypto/curve.go is not our code and has its own license. This commit
excludes it in update-license.go and removes our GPL header.

This version is less clever. All names are listed in a single file,
AUTHORS. All source files have the same header. This is an improvement
over the previous version, which attempted to list copyright holders in
each source file.

This ensures that compiler error messages contain the correct path.

Many people need or want to build go-ethereum from the git repository,
mostly to stay up to date with recent changes. We cannot expect that
people without Go experience grok the Go workspace concept.

With the Makefile, building from github requires only
three steps (provided that a Go toolchain is installed):

    - git clone https://github.com/ethereum/go-ethereum
    - ... install C libraries (libgmp, etc.) ...
    - make