There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.
This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.
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.
