Fix #1 build errors + gofmt

Both `Ledgers` and `HaveList` should call `Multihash.HexString()` in order to set elements of maps because type Multihash has `[]byte` as underlying type (instead of struct) and can't be used as map key.

```go
type BitSwap struct {
	Ledgers  map[string]*Ledger
	HaveList map[string]*blocks.Block
	WantList []*multihash.Multihash
}
```

EDIT: Do you want me to split this into two commits. One for type changes and another one for gofmt?