go: Encoder.version isn't exported, we don't need to validate it
go: only set compressed document type if body is compressed

This is a revert (+fixes) of 20500a29d15c2e15583f87a6a348109c0afb48b6.
go: optionally allow using cgo bindings for snappy and zlib

This patch adds a build tag 'clibs' which will link against C
implementations of snappy and zlib instead of the pure Go ones.  The Go
versions are still built and used by default.

Note that binaries built with sereal linked against the C libraries must
have compatible C libraries available at runtime -- they are no longer
standalone.
go: update README with build instructions for cgo compression libs
Merge branch 'master' into kruglov/merger
merge.go: fix broken tests
merge.go: merger's output can be compressed by snappy or zlib