Initial impl. of cloud provider for AWS

Sumitting the pull request discussed in issue https://github.com/GoogleCloudPlatform/kubernetes/issues/1187 for initial review and more feedback.

Implements only the `cloudprovider.Instances` interface for now.

**New dependencies**:
- https://github.com/mitchellh/goamz: for talking to AWS API
- https://code.google.com/p/gcfg/: for reading cloud provider configuration (as suggested in issue)

**Caveats**:
- DNS name used is private DNS names (kubernetes should run in VPC)
- filtering is done on the `Name` tag (private DNS is auto-assigned and not something you can affect)
  - should move away from regexp in order to better leverage cloud provider specific filtering capabilities (tags for AWS)

Configuration file is in INI format (as implemented by gcfg) and currently has only one (required) configuration option, the region:

```
[global]
region = eu-west-1
```