ec2: add BlockDevice support for Instances() call

This was missing in the response so there was no way to obtain it. We
can't use the BlockDeviceMapping struct because the response in the
InstancesResp is different than the others. So we add a new
`BlockDevice` struct that represents the Block device response coming
from the instances resp.  The response information can be found here:

http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeInstances.html

In the form of

```
block-device-mapping.attach-time
The attach time for an Amazon EBS volume mapped to the instance (for
example, 2010-09-15T17:15:20.000Z)

Type: DateTime

block-device-mapping.delete-on-termination
Indicates whether the Amazon EBS volume is deleted on instance
termination.

Type: Boolean

block-device-mapping.device-name
The device name (for example, /dev/sdh) for the Amazon EBS volume.

Type: String

block-device-mapping.status
The status for the Amazon EBS volume.

Type: String

Valid values: attaching | attached | detaching | detached

block-device-mapping.volume-id
The volume ID of the Amazon EBS volume.

Type: String
```

The test response was already containing the in the form of

```xml
          <blockDeviceMapping>
            <item>
              <deviceName>/dev/sda1</deviceName>
              <ebs>
                <volumeId>vol-a082c1c9</volumeId>
                <status>attached</status>
                <attachTime>2010-08-17T01:15:21.000Z</attachTime>
                <deleteOnTermination>false</deleteOnTermination>
              </ebs>
            </item>
          </blockDeviceMapping>
```

Therefore I also included this missing case to the tests.
Merge pull request #99 from fatih/add-block-device

ec2: add missing BlockDevice support for Instances() method
Merge pull request #98 from savaki/master

- added support for DescribeTags
autoscaling: Fix launch-config test

The library takes care of b64 encoding it,
but the test wasn't updated to reflect that.
Merge pull request #100 from ambakshi/autoscaling/fix-launch-config-test

autoscaling: Fix launch-config test
sdb: Fix sign() to pass auth.Token as []string
Merge pull request #101 from ambakshi/sdb/fix-sdb-test

sdb: Fix sign() to pass auth.Token as []string
autoscaling: Add support for TerminationPolicies

see: http://docs.aws.amazon.com/AutoScaling/latest/DeveloperGuide/us-termination-policy.html
Merge pull request #102 from ambakshi/autoscaling/termination-policies

autoscaling: Add support for TerminationPolicies
aws.Auth: Check for AWS_CREDENTIAL_FILE env

The location of the credentials file was hardcoded
to ~/.aws/credentials.

see: http://docs.aws.amazon.com/IAM/latest/CLIReference/Options.html
autoscaling: Fix TerminationPolicies serialization
Add GoDoc badge
Extracting only one instance from DescribeLoadBalancersResponse when the load balancer has more than one
Merge pull request #105 from fatih/patch-1

Add GoDoc badge
Merge pull request #106 from tegioz/master

Extracting only one instance from DescribeLoadBalancersResponse when the load balancer has more than one
Merge pull request #104 from ambakshi/autoscaling/fix-termination-policies

autoscaling: Fix TerminationPolicies serialization
Merge pull request #103 from ambakshi/auth/check-credentials-env

aws.Auth: Check for AWS_CREDENTIAL_FILE env
Merge pull request #107 from savaki/master

Added support for AddTags and RemoveTags