Add segment create and read

[zhouzhuojie]

GET localhost:18000/api/flags/1/segments

[
	{
		"constraints": null,
		"description": "segment_1",
		"distributions": null,
		"id": 1,
		"rank": 0
	},
	{
		"constraints": null,
		"description": "segment_2",
		"distributions": null,
		"id": 2,
		"rank": 1
	}
]

localhost:18000/api/flags/1/segments

{
	"constraints": null,
	"description": "segment_2",
	"distributions": null,
	"id": 2
}