adds owners argument to Images() as ImagesByOwner()
ec2: Add VirtType parameter to RegisterImage

It's now possible to create instance-store HVM AMIs:

    http://sorcery.smugmug.com/2014/01/29/instance-store-hvm-amis-for-amazon-ec2/

This is controlled by a new optional parameter `VirtType` to
`RegisterImage`, which must be either `paravirtual` or `hvm`.
Merge pull request #31 from LessThanThreeLabs/master

Adds owners argument to Images() as ImagesByOwner()
Add volume tags to DescribeVolumes response
Added ModifyInstanceAttribute API call.
Merge pull request #34 from jmassara/modifyInstance

Add ModifyInstanceAttribute API call.
Merge pull request #32 from grosskur/add-virt-type-to-register-image

ec2: Add VirtType parameter to RegisterImage
Adding tags to the DescribeVolumes response
Merge pull request #35 from jmassara/volTags

Adding tags to the DescribeVolumes response
Include x-amz-security-token in s3 during signing

To quote the documentation:
```
If you are signing your request using temporary security credentials (see Making Requests),
you must include the corresponding security token in your request by adding the
x-amz-security-token header.
```

It wasn't included in headers before, and s3 couldn't be accessed with
the temporary token. Luckily this commit fixes that.
Merge pull request #36 from libc/s3_iam_fix_signing

Include x-amz-security-token in s3 during signing
do not close response until buildError finishes

`s3_test#TestGetNotFound` failed because it's trying to decode hresp.Body, but
the body's already closed. This patch defers `hresp.Body.Close()` until it
finishes building the error.
Merge pull request #37 from nitrous-io/fix-get-not-found-test

do not close response until buildError finishes
add bucket.GetKey to get metadata of a key
Infer the NextMarker if S3 didn't send it

Otherwise this only works if you specified a delimiter in the request.
The symptom is an infinite loop endlessly processing the exact set of
keys over and over again.
Merge pull request #39 from bobveznat/master

Infer the NextMarker if S3 didn't send it
Merge pull request #38 from nitrous-io/bucket-get-key

add bucket.GetKey to get metadata of a key
Add the support for "RequestSpotInstances" and "DescribeSpotInstanceRequests"
Merge pull request #41 from henrysher/add-spot-instance-support

add the support for spot instances
Add the API "CancelSpotRequests"