add unix datagram socket support

[vektah]

This change allows the caller to set the network type, which allows for broader compatibility, eg with stripe/veneur unix abstract domain sockets

statsd.NewClient(
   "@veneur-socket",
    statsd.Network("unixgram"),
    statsd.TagStyle(statsd.TagFormatDatadog),
)

No change to existing behaviour. UDP is still the default.

No perf changes

BenchmarkSimple-12                      7402266               148.5 ns/op
BenchmarkSimpleUnixSocket-12            8521453               140.6 ns/op
BenchmarkComplexDelivery-12             5854812               196.4 ns/op
BenchmarkTagged-12                      2821629               423.7 ns/op
PASS
ok      github.com/smira/go-statsd      9.744s

[smira]

Thanks for your PR, but I guess I need to fix the CI first.