Added support for credential files and credential profiles

Added support for credential files (INI format).

Your credential file should be stored in `~/.aws/credential`, and look like:

```ini
[default]
aws_access_key_id=key1
aws_secret_access_key=secret1

[profile1]
aws_access_key_id=key2
aws_secret_access_key=secret2

[profile2]
aws_access_key_id=key3
aws_secret_access_key=secret3
```

/cc @mtdowling