This pulls in multilog support (logs sharded by date). As a result,
it also pulls in new dependencies gogo/protobuf (for UnmarshalText) and
golang/protobuf/ptypes (for Timestamp).
Replaces #3202, adding a smaller set of dependencies. See also #3205.
Tests run:
```
$ go test github.com/gogo/protobuf/proto github.com/golang/protobuf/ptypes/... github.com/google/certificate-transparency-go/...
ok github.com/gogo/protobuf/proto 0.063s
ok github.com/golang/protobuf/ptypes 0.009s
? github.com/golang/protobuf/ptypes/any [no test files]
? github.com/golang/protobuf/ptypes/duration [no test files]
? github.com/golang/protobuf/ptypes/empty [no test files]
? github.com/golang/protobuf/ptypes/struct [no test files]
? github.com/golang/protobuf/ptypes/timestamp [no test files]
? github.com/golang/protobuf/ptypes/wrappers [no test files]
ok github.com/google/certificate-transparency-go 1.005s
ok github.com/google/certificate-transparency-go/asn1 0.021s
ok github.com/google/certificate-transparency-go/client 22.034s
? github.com/google/certificate-transparency-go/client/ctclient [no test files]
ok github.com/google/certificate-transparency-go/fixchain 0.145s
? github.com/google/certificate-transparency-go/fixchain/main [no test files]
ok github.com/google/certificate-transparency-go/fixchain/ratelimiter 27.745s
ok github.com/google/certificate-transparency-go/gossip 0.772s
? github.com/google/certificate-transparency-go/gossip/main [no test files]
ok github.com/google/certificate-transparency-go/jsonclient 25.523s
ok github.com/google/certificate-transparency-go/merkletree 0.004s
? github.com/google/certificate-transparency-go/preload [no test files]
? github.com/google/certificate-transparency-go/preload/dumpscts/main [no test files]
? github.com/google/certificate-transparency-go/preload/main [no test files]
ok github.com/google/certificate-transparency-go/scanner 0.010s
? github.com/google/certificate-transparency-go/scanner/main [no test files]
ok github.com/google/certificate-transparency-go/tls 0.026s
ok github.com/google/certificate-transparency-go/x509 0.417s
? github.com/google/certificate-transparency-go/x509/pkix [no test files]
? github.com/google/certificate-transparency-go/x509util [no test files]
```