Adding network interface information and security group ids

Updating describe_instances call.

Amazon API return the following:

```xml
<item>
    <instanceId>i-XXXXX</instanceId>
    <imageId>ami-XXXXX</imageId>
    <instanceState>
        <code>16</code>
        <name>running</name>
    </instanceState>
    <privateDnsName/>
    <dnsName/>
    <reason/>
    <keyName>key</keyName>
    <amiLaunchIndex>0</amiLaunchIndex>
    <productCodes/>
    <instanceType>c1.medium</instanceType>
    <launchTime>2012-05-06T19:32:29.000Z</launchTime>
    <placement>
        <availabilityZone>us-west-2a</availabilityZone>
        <groupName/>
        <tenancy>default</tenancy>
    </placement>
    <kernelId>aki-XXXXXX</kernelId>
    <monitoring>
        <state>disabled</state>
    </monitoring>
    <subnetId>subnet-XXXXXX</subnetId>
    <vpcId>vpc-XXXXX</vpcId>
    <privateIpAddress>111.111.111.111</privateIpAddress>
    <ipAddress>222.222.222.222</ipAddress>
    <sourceDestCheck>false</sourceDestCheck>
    <groupSet>
        <item>
            <groupId>sg-XXXXXXX</groupId>
            <groupName>prd-vpn</groupName>
        </item>
    </groupSet>
    <architecture>x86_64</architecture>
    <rootDeviceType>ebs</rootDeviceType>
    <rootDeviceName>/dev/sda1</rootDeviceName>
    <blockDeviceMapping>
        <item>
            <deviceName>/dev/sda1</deviceName>
            <ebs>
                <volumeId>vol-XXXXXXX</volumeId>
                <status>attached</status>
                <attachTime>2012-05-06T19:33:01.000Z</attachTime>
                <deleteOnTermination>true</deleteOnTermination>
            </ebs>
        </item>
    </blockDeviceMapping>
    <virtualizationType>paravirtual</virtualizationType>
    <clientToken/>
    <tagSet>
        <item>
            <key>Name</key>
            <value>vpc-vpn2</value>
        </item>
    </tagSet>
    <hypervisor>xen</hypervisor>
    <networkInterfaceSet>
        <item>
            <networkInterfaceId>eni-XXXXXX</networkInterfaceId>
            <subnetId>subnet-XXXXXX</subnetId>
            <vpcId>vpc-XXXXXX</vpcId>
            <description>Primary network interface</description>
            <ownerId>XXXXXXXXXX</ownerId>
            <status>in-use</status>
            <privateIpAddress>111.111.111.111</privateIpAddress>
            <sourceDestCheck>false</sourceDestCheck>
            <groupSet>
                <item>
                    <groupId>sg-XXXXXXXX</groupId>
                    <groupName>prd-vpn</groupName>
                </item>
            </groupSet>
            <attachment>
                <attachmentId>eni-attach-XXXXXX</attachmentId>
                <deviceIndex>0</deviceIndex>
                <status>attached</status>
                <attachTime>2012-05-06T19:32:29.000Z</attachTime>
                <deleteOnTermination>false</deleteOnTermination>
            </attachment>
            <association>
                <publicIp>222.222.222.222</publicIp>
                <ipOwnerId>XXXXXXXX</ipOwnerId>
            </association>
        </item>
    </networkInterfaceSet>
</item>
```

This pull request adds parsing of the networkInterfaceSet tag and elements.
This pull request also moves security GroupIds into an array.

On a practical side, knife ec2 output before the patch:

```

knife ec2 server list
Instance ID  Public IP       Private IP     Flavor     Image         SSH Key  Security Groups                State  
i-XXXXXXXX   111.111.111.111   111.111.111.111  m1.large   ami-XXXXXXXX           sg-XXXXXXXX, web               running
i-XXXXXXXX   111.111.111.111   111.111.111.111  m1.large   ami-XXXXXXXX           sg-XXXXXXXX, web               running
i-XXXXXXXX   111.111.111.111   111.111.111.111  c1.medium  ami-XXXXXXXX  artem                                   running
                                            t1.micro                                                         [{}]   
i-XXXXXXXX                   111.111.111.111     c1.medium  ami-XXXXXXXX  artem                                   running
                                            t1.micro                                                         [{}]   
i-XXXXXXXX                   111.111.111.111    c1.medium  ami-XXXXXXXX  artem                                   running
                                            t1.micro                                                         [{}]   
i-XXXXXXXX   111.111.111.111  111.111.111.111    c1.medium  ami-XXXXXXXX  artem                                   running
                                            t1.micro                                                         [{}]   

```

Notice the empty "t1.micro  [{}]" instance objects created because of parsing failures for VPC instances.

The pull request addresses this issue.