Add SSL and basic auth support in API

Add support to API for both SSL and basic auth.

To activate basic auth, set the `Username` and/or `Password` on the Api like this:

```
	api := gobot.Api(master)
	api.Username = "gort"
	api.Password = "klatuu"
```

To activate SSL, set the `Cert` and `Key` on the Api like this:

```
	api := gobot.Api(master)
	api.Cert = "/path/to/cert"
	api.Key = "/path/to/key"
```