modules/coin/rest: implemented CreateRole

* Note: Role must be a hex string, as enforced in tests/rest/cli.sh

```shell
$ curl -X POST http://localhost:8998/build/create_role --data \
'{
  "role":"DEADBEEF", "seq": 1,
  "min_sigs": 1,
  "signers": [{
    "addr": "4FF759D47C81754D8F553DCCAC8651D0AF74C7F9", "app": "role"
  }]
}'
```

```HTTP
HTTP/1.1 200 OK
Content-Type: application/json
Date: Tue, 08 Aug 2017 19:15:13 GMT
Content-Length: 387

{
  "type": "chain/tx",
  "data": {
    "chain_id": "test_chain_id",
    "expires_at": 0,
    "tx": {
      "type": "role/create",
      "data": {
        "role": "DEADBEEF",
        "min_sigs": 1,
        "signers": [
          {
            "chain": "",
            "app": "role",
            "addr": "4FF759D47C81754D8F553DCCAC8651D0AF74C7F9"
          }
        ]
      }
    }
  }
}
```

Updates #200

modules/roles/rest/handlers.go

@@ -0,0 +1,74 @@
+package rest
+
+import (
+	"encoding/hex"
+	"net/http"
+
+	"github.com/gorilla/mux"
+
+	abci "github.com/tendermint/abci/types"
+	"github.com/tendermint/basecoin"
+	"github.com/tendermint/basecoin/client/commands"
+	"github.com/tendermint/basecoin/errors"
+	"github.com/tendermint/basecoin/modules/base"
+	"github.com/tendermint/basecoin/modules/nonce"
+	"github.com/tendermint/basecoin/modules/roles"
+	"github.com/tendermint/tmlibs/common"
+)
+
+type RoleInput struct {
+	Role string `json:"role" validate:"required,min=2"`
+
+	MinimumSigners uint32 `json:"min_sigs" validate:"required,min=1"`
+
+	Signers []basecoin.Actor `json:"signers" validate:"required,min=1"`
+
+	Sequence uint32 `json:"seq" validate:"required,min=1"`
+}
+
+func parseRole(roleInHex string) ([]byte, error) {
+	parsedRole, err := hex.DecodeString(common.StripHex(roleInHex))
+	if err != nil {
+		err = errors.WithMessage("invalid hex", err, abci.CodeType_EncodingError)
+		return nil, err
+	}
+	return parsedRole, nil
+}
+
+// mux.Router registrars
+
+// RegisterQueryAccount is a mux.Router handler that exposes GET
+// method access on route /query/account/{signature} to query accounts
+func RegisterCreateRole(r *mux.Router) error {
+	r.HandleFunc("/build/create_role", doCreateRole).Methods("POST")
+	return nil
+}
+
+func doCreateRole(w http.ResponseWriter, r *http.Request) {
+	ri := new(RoleInput)
+	if err := common.ParseRequestAndValidateJSON(r, ri); err != nil {
+		common.WriteError(w, err)
+		return
+	}
+
+	parsedRole, err := parseRole(ri.Role)
+	if err != nil {
+		common.WriteError(w, err)
+		return
+	}
+
+	// Note the ordering of Tx wrapping matters:
+	// 1. NonceTx
+	tx := (nonce.Tx{}).Wrap()
+	tx = nonce.NewTx(ri.Sequence, ri.Signers, tx)
+
+	// 2. CreateRoleTx
+	tx = roles.NewCreateRoleTx(parsedRole, ri.MinimumSigners, ri.Signers)
+
+	// 3. ChainTx
+	tx = base.NewChainTx(commands.GetChainID(), 0, tx)
+
+	common.WriteSuccess(w, tx)
+}
+
+// End of mux.Router registrars