Added new API route for retrieving available device graphs

Call:

curl -H "X-Auth-Token: APITOKEN" http://librenms.org/api/v0/devices/localhost/graphs

Output:

{
    "status": "ok",
    "err-msg": "",
    "count": 25,
    "graphs": [
        {
            "desc": "Poller Time",
            "name": "device_poller_perf"
        },
        {
            "desc": "Ping Response",
            "name": "device_ping_perf"
        },
        {
            "desc": "Agent Execution Time",
            "name": "agent"
        },
        {
            "desc": "Running Processes",
            "name": "hr_processes"
        },
        {
            "desc": "Users Logged In",
            "name": "hr_users"
        },
        {
            "desc": "IPv4 Packet Statistics",
            "name": "ipsystemstats_ipv4"
        },
        {
            "desc": "IPv4 Fragmentation Statistics",
            "name": "ipsystemstats_ipv4_frag"
        },
        {
            "desc": "IPv6 Packet Statistics",
            "name": "ipsystemstats_ipv6"
        },
        {
            "desc": "IPv6 Fragmentation Statistics",
            "name": "ipsystemstats_ipv6_frag"
        },
        {
            "desc": "ICMP Statistics",
            "name": "netstat_icmp"
        },
        {
            "desc": "ICMP Informational Statistics",
            "name": "netstat_icmp_info"
        },
        {
            "desc": "IP Statistics",
            "name": "netstat_ip"
        },
        {
            "desc": "IP Fragmentation Statistics",
            "name": "netstat_ip_frag"
        },
        {
            "desc": "SNMP Statistics",
            "name": "netstat_snmp"
        },
        {
            "desc": "SNMP Packet Type Statistics",
            "name": "netstat_snmp_pkt"
        },
        {
            "desc": "TCP Statistics",
            "name": "netstat_tcp"
        },
        {
            "desc": "UDP Statistics",
            "name": "netstat_udp"
        },
        {
            "desc": "Context Switches",
            "name": "ucd_contexts"
        },
        {
            "desc": "Detailed Processor Usage",
            "name": "ucd_cpu"
        },
        {
            "desc": "Interrupts",
            "name": "ucd_interrupts"
        },
        {
            "desc": "System I/O Activity",
            "name": "ucd_io"
        },
        {
            "desc": "Load Averages",
            "name": "ucd_load"
        },
        {
            "desc": "Detailed Memory Usage",
            "name": "ucd_memory"
        },
        {
            "desc": "Swap I/O Activity",
            "name": "ucd_swap_io"
        },
        {
            "desc": "System Uptime",
            "name": "uptime"
        }
    ]
}