refactor(util) move block generator

@whyrusleeping @jbenet

Putting the block generator in a util dir until blocks.

Can't put it in util/testutil because the util/testutil/dag-generator
imports blockservice and blockservice uses the generator.

Tough problem. This'll do for now.

License: MIT
Signed-off-by: Brian Tiger Chow <brian@perfmode.com>

block_generator.go

+package blocksutil
+
+import "github.com/jbenet/go-ipfs/blocks"
+
+func NewBlockGenerator() BlockGenerator {
+	return BlockGenerator{}
+}
+
+type BlockGenerator struct {
+	seq int
+}
+
+func (bg *BlockGenerator) Next() *blocks.Block {
+	bg.seq++
+	return blocks.NewBlock([]byte(string(bg.seq)))
+}
+
+func (bg *BlockGenerator) Blocks(n int) []*blocks.Block {
+	blocks := make([]*blocks.Block, 0)
+	for i := 0; i < n; i++ {
+		b := bg.Next()
+		blocks = append(blocks, b)
+	}
+	return blocks
+}