Fix to avoid the '_msearchTimer of undefined' error

Currently, if you try to use your library following the instructions on the README
```javascript
var Ssdp = require('upnp-ssdp');
var server = Ssdp();
```
it will throw an error like this:

```bash
/Users/satanas/proyectos/node/quantic/node_modules/upnp-ssdp/index.js:52
    this._msearchTimer = null;
                       ^
TypeError: Cannot set property '_msearchTimer' of undefined
```

That's because the context of the Ssdp object has been not set by the moment of creation. With this fix the context is set at the beginning so everything works as expected.

Other solution would be to modify the README and indicate explicitly that you new to use **new** when you create a Ssdp instance:

```javascript
var Ssdp = require('upnp-ssdp');
var server = new Ssdp();
```