Allow no SSL certificate verify as param

Just a conveniency to let non valid SSL certs (either because self-signed or wildcard) to go along without providing a full `session` object in the args.

I'm not sure what is the original purpose of allowing `session` to be passed as arg to the instantiation of the `ZabbixAPI` class, so it may duplicate its use.

I agree we could simply use the following code:

```python
from requests import Session
from pyzabbix import ZabbixAPI
s = Session()
s.verify = False
zapi = ZabbixAPI(server='https://my.zabbix.com', session=s)
```

But I believe it's somehow simpler to just go with:

```python
from pyzabbix import ZabbixAPI
zapi = ZabbixAPI(server='https://my.zabbix.com', ssl_verify=False)
```

I let it to your best judgement :)