change jaeger options to functional style

[paivagustavo]

Fixes #146

With this i have separated the Options struct in 3:

1. General Options that apply to any jaeger exporter;
2. Collector Endpoint Options;
3. Agent Endpoint Options

Every exporter must pass one, and only one, endpoint (through the EndpointOption). The new signature is:

func NewExporter(endpointOption EndpointOption, opts ...Option) (*Exporter, error) {

Usage examples:

	exporter, err := jaeger.NewExporter(
		jaeger.WithCollectorEndpoint("http://localhost:14268/api/traces"),
		jaeger.WithProcess(jaeger.Process{
			ServiceName: "trace-demo",
		}),
	)

	exporter, err = jaeger.NewExporter(
		jaeger.WithCollectorEndpoint(
			"http://localhost:14268/api/traces",
			jaeger.WithUsername("user"), // this only exists inside "WithCollectorEndpoint" option
			jaeger.WithPassword("password")), // this only exists inside "WithCollectorEndpoint" option
		jaeger.WithOnError(func(err error) {
			log.Fatal(err.Error())
		}),
	)

	exporter, err = jaeger.NewExporter(
		jaeger.WithAgentEndpoint("localhost:6831"),
		jaeger.WithBufferMaxCount(100),
	)

I have also extracted the upload to an interface:

// batchUploader send a batch of spans to Jaeger
type batchUploader interface {
	upload(batch *gen.Batch) error
}

The EndpointOption generates an implementation of batchUploader based on the option passed by the user.

[paivagustavo]

@rghetia I've changed what you request, could you review it again?