Update ZabbixAPI.cs

1)
after Problems with authentication to Zabbix 2.4.1...

use Query-Object without Param "auth" for API-Calls with method apiinfo.version" || "user.login"

see: Zabbix Documentation 2.0
apiinfo.version
string apiinfo.version(array)
 
"This method is available to unauthenticated users and should be called without the auth parameter in the JSON-RPC request. Starting from Zabbix 2.4 the method will return an error if the auth parameter is given."

2)
Issue #4 missing host name:    on alerts the host field is null.
the Parameter "select_hosts" seems to have been renamed to "selectHosts"

change all parameter definitions from select_hosts to selectHosts

see: 
Zabbix Documentation 2.0
trigger.get
integer/array trigger.get(object parameters