Add the /register endpoint

## POST /v1/msisdn/register

Starts the registration of a MSISDN in E.164 format. This will trigger a verification code to be sent to the MSISDN via SMS.

___Parameters___
* msisdn - a MSISDN in E.164 format

### Request

```sh
curl -v \
-X POST \
-H "Content-Type: application/json" \
"https://api.accounts.firefox.com/v1/msisdn/register" \
-d '{
  "msisdn": "+442071838750"
}'
```

### Response

Successful requests will produce a "200 OK" response with following format:

```json
{
  "msisdnSessionToken": "27cd4f4a4aa03d7d186a2ec81cbf19d5c8a604713362df9ee15c4f4a4aa03d7d"
}
```