EC2 additions and name corrections

Updates `ec2` with additional parameters on the `Instance` type, and make these match the Amazon API (rather than having custom names for fields and methods). Unfortunately this causes some backwards compatibility issues, including:

`ec2.Instances` method (which called _DescribeInstances_) called `ec2.DescribeInstances`
`ec2.InstancesResp` type (response from _DescribeInstances_) now called `ec2.DescribeInstancesResp`
`ec2.Instance.AvailZone` now called `AvailabilityZone`
`ec2.Instance.VirtType` now called `VirtualizationType`
`ec2.Instance.IamInstanceProfile` is now of type `ec2.IamInstanceProfile` rather than `string`


The `ec2.Instance` type now has all of the API parameters implemented, including:
```go
Reason                string
StateReason           InstanceStateReason
LaunchTime            string
RootDeviceType        string
RootDeviceName        string
EbsOptimized          bool
OwnerId               string
Tenancy               string
SecurityGroups        []SecurityGroup
Architecture          string
KernelId              string
RamDiskId             string
Platform              string
ClientToken           string
ProductCodes          []ProductCode
```

Also methods for spot instance types:
```go
InstanceLifecycle     string
SpotInstanceRequestId string

func (i Instance) isSpotInstance() bool {}
```

Added more advanced networking information which lists network interfaces, attachments, private IP assignments, and elastic IP attachments:

```go
NetworkInterfaces []InstanceNetworkInterface
SourceDestCheck   bool
SriovNetSupport   string
```

Probably a bit more work to be done on this, but thoughts would be appreciated.