Added new API route for billing information

Calls:

curl -H "X-Auth-Token: 91c60e737e342c205be5bba8e2954d27" \
"https://librenms.example.com/api/v0/bills"

curl -H "X-Auth-Token: 91c60e737e342c205be5bba8e2954d27" \
"https://librenms.example.com/api/v0/bills/$bill_id"

Output:

{
    "status": "ok",
    "err-msg": "",
    "count": 2,
    "bills": [
        {
            "bill_id": "1",
            "bill_name": "Testing",
            "bill_type": "cdr",
            "bill_cdr": "10001000000000",
            "bill_day": "1",
            "bill_quota": "0",
            "rate_95th_in": "22818",
            "rate_95th_out": "56652",
            "rate_95th": "56652",
            "dir_95th": "out",
            "total_data": "926085445",
            "total_data_in": "411520932",
            "total_data_out": "514564513",
            "rate_average_in": "0",
            "rate_average_out": "0",
            "rate_average": "43366",
            "bill_last_calc": "2014-12-02 23:01:03",
            "bill_custid": "Testing",
            "bill_ref": "testref",
            "bill_notes": "",
            "bill_autoadded": "0"
        },
        {
            "bill_id": "2",
            "bill_name": "W00t",
            "bill_type": "cdr",
            "bill_cdr": "100000000",
            "bill_day": "1",
            "bill_quota": "0",
            "rate_95th_in": "0",
            "rate_95th_out": "0",
            "rate_95th": "0",
            "dir_95th": "",
            "total_data": "0",
            "total_data_in": "0",
            "total_data_out": "0",
            "rate_average_in": "0",
            "rate_average_out": "0",
            "rate_average": "0",
            "bill_last_calc": "0000-00-00 00:00:00",
            "bill_custid": "Nowbown",
            "bill_ref": "Sadsadas",
            "bill_notes": "",
            "bill_autoadded": "0"
        }
    ]
}

{
    "status": "ok",
    "err-msg": "",
    "count": 1,
    "bills": [
        {
            "bill_id": "2",
            "bill_name": "W00t",
            "bill_type": "cdr",
            "bill_cdr": "100000000",
            "bill_day": "1",
            "bill_quota": "0",
            "rate_95th_in": "0",
            "rate_95th_out": "0",
            "rate_95th": "0",
            "dir_95th": "",
            "total_data": "0",
            "total_data_in": "0",
            "total_data_out": "0",
            "rate_average_in": "0",
            "rate_average_out": "0",
            "rate_average": "0",
            "bill_last_calc": "0000-00-00 00:00:00",
            "bill_custid": "Nowbown",
            "bill_ref": "Sadsadas",
            "bill_notes": "",
            "bill_autoadded": "0"
        }
    ]
}