add abstraction for HLR service

Adds API for HLR service.

I didn't quite knew if you require some checks to be performed on input in *Action class' methods.

```python
api = SmsAPI()

api.set_username('USERNAME')
api.set_password('PASSWORD')

try:
    api.service('hlr').action('check')
    api.set_numbers('NUM1', 'NUM2')

    result = api.execute()
    for r in result:
        print r.status, r.price, r.number, r.id

except ApiError, e:
    print '%s - %s' % (e.code, e.message)
```